Make the hangup button do things for conference calls

Behaviour constraints:
* If you're not in the conference, use a grey button that does nothing.
* If you're in the conference, show a button:
  * If you're able to modify widgets in the room, annotate it in the context of ending the call for everyone and remove the widget. Use a confirmation dialog.
  * If you're not able to modify widgets in the room, hang up.

For this we know that persistent Jitsi widgets will mean that the user is in the call, so we use that to determine if they are actually participating.

src/components/views/rooms/MessageComposer.js

@@ -1,6 +1,7 @@
 /*
 Copyright 2015, 2016 OpenMarket Ltd
 Copyright 2017, 2018 New Vector Ltd
+Copyright 2020 The Matrix.org Foundation C.I.C.
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -32,6 +33,10 @@ import {aboveLeftOf, ContextMenu, ContextMenuTooltipButton, useContextMenu} from
 import AccessibleTooltipButton from "../elements/AccessibleTooltipButton";
 import ReplyPreview from "./ReplyPreview";
 import {UIFeature} from "../../../settings/UIFeature";
+import WidgetStore from "../../../stores/WidgetStore";
+import WidgetUtils from "../../../utils/WidgetUtils";
+import {UPDATE_EVENT} from "../../../stores/AsyncStore";
+import ActiveWidgetStore from "../../../stores/ActiveWidgetStore";
 
 function ComposerAvatar(props) {
     const MemberStatusMessageAvatar = sdk.getComponent('avatars.MemberStatusMessageAvatar');
@@ -85,8 +90,15 @@ VideoCallButton.propTypes = {
 };
 
 function HangupButton(props) {
-    const AccessibleButton = sdk.getComponent('elements.AccessibleButton');
     const onHangupClick = () => {
+        if (props.isConference) {
+            dis.dispatch({
+                action: props.canEndConference ? 'end_conference' : 'hangup_conference',
+                room_id: props.roomId,
+            });
+            return;
+        }
+
         const call = CallHandler.getCallForRoom(props.roomId);
         if (!call) {
             return;
@@ -98,14 +110,28 @@ function HangupButton(props) {
             room_id: call.roomId,
         });
     };
-    return (<AccessibleButton className="mx_MessageComposer_button mx_MessageComposer_hangup"
+
+    let tooltip = _t("Hangup");
+    if (props.isConference && props.canEndConference) {
+        tooltip = _t("End conference");
+    }
+
+    const canLeaveConference = !props.isConference ? true : props.isInConference;
+    return (
+        <AccessibleTooltipButton
+            className="mx_MessageComposer_button mx_MessageComposer_hangup"
             onClick={onHangupClick}
-            title={_t('Hangup')}
-        />);
+            title={tooltip}
+            disabled={!canLeaveConference}
+        />
+    );
 }
 
 HangupButton.propTypes = {
     roomId: PropTypes.string.isRequired,
+    isConference: PropTypes.bool.isRequired,
+    canEndConference: PropTypes.bool,
+    isInConference: PropTypes.bool,
 };
 
 const EmojiButton = ({addEmoji}) => {
@@ -226,12 +252,17 @@ export default class MessageComposer extends React.Component {
         this._onRoomViewStoreUpdate = this._onRoomViewStoreUpdate.bind(this);
         this._onTombstoneClick = this._onTombstoneClick.bind(this);
         this.renderPlaceholderText = this.renderPlaceholderText.bind(this);
+        WidgetStore.instance.on(UPDATE_EVENT, this._onWidgetUpdate);
+        ActiveWidgetStore.on('update', this._onActiveWidgetUpdate);
         this._dispatcherRef = null;
+
         this.state = {
             isQuoting: Boolean(RoomViewStore.getQuotingEvent()),
             tombstone: this._getRoomTombstone(),
             canSendMessages: this.props.room.maySendMessage(),
             showCallButtons: SettingsStore.getValue("showCallButtonsInComposer"),
+            hasConference: WidgetStore.instance.doesRoomHaveConference(this.props.room),
+            joinedConference: WidgetStore.instance.isJoinedToConferenceIn(this.props.room),
         };
     }
 
@@ -247,6 +278,14 @@ export default class MessageComposer extends React.Component {
         }
     };
 
+    _onWidgetUpdate = () => {
+        this.setState({hasConference: WidgetStore.instance.doesRoomHaveConference(this.props.room)});
+    };
+
+    _onActiveWidgetUpdate = () => {
+        this.setState({joinedConference: WidgetStore.instance.isJoinedToConferenceIn(this.props.room)});
+    };
+
     componentDidMount() {
         this.dispatcherRef = dis.register(this.onAction);
         MatrixClientPeg.get().on("RoomState.events", this._onRoomStateEvents);
@@ -277,6 +316,8 @@ export default class MessageComposer extends React.Component {
         if (this._roomStoreToken) {
             this._roomStoreToken.remove();
         }
+        WidgetStore.instance.removeListener(UPDATE_EVENT, this._onWidgetUpdate);
+        ActiveWidgetStore.removeListener('update', this._onActiveWidgetUpdate);
         dis.unregister(this.dispatcherRef);
     }
 
@@ -392,9 +433,19 @@ export default class MessageComposer extends React.Component {
             }
 
             if (this.state.showCallButtons) {
-                if (callInProgress) {
+                if (this.state.hasConference) {
+                    const canEndConf = WidgetUtils.canUserModifyWidgets(this.props.room.roomId);
                     controls.push(
-                        <HangupButton key="controls_hangup" roomId={this.props.room.roomId} />,
+                        <HangupButton
+                            roomId={this.props.room.roomId}
+                            isConference={true}
+                            canEndConference={canEndConf}
+                            isInConference={this.state.joinedConference}
+                        />,
+                    );
+                } else if (callInProgress) {
+                    controls.push(
+                        <HangupButton key="controls_hangup" roomId={this.props.room.roomId} isConference={false} />,
                     );
                 } else {
                     controls.push(