Implement call hold

Currently just by adding /holdcall and /unholdcall slash commands

The only place the hold status of the call is currently represented
is when the call is a voice call and you're viewing a different room:
it's not wired up when you're viewing the room because that currently
uses the room status bar which it won't do with the new UI.

Also convert VideoFeed to typescript, and remove videoview because
it essentially just managed the fullscreen functionality, but we'll
want and 'on hold' representation (and probably chrome for hagnup etc)
in the fullscreen UI too, so let's just make CallView the thing that
gets fullscreened.

src/components/views/voip/VideoFeed.tsx

@@ -0,0 +1,82 @@
+/*
+Copyright 2015, 2016 OpenMarket Ltd
+Copyright 2019 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.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import classnames from 'classnames';
+import { MatrixCall } from 'matrix-js-sdk/src/webrtc/call';
+import React, {createRef} from 'react';
+import SettingsStore from "../../../settings/SettingsStore";
+
+export enum VideoFeedType {
+    Local,
+    Remote,
+}
+
+interface IProps {
+    call: MatrixCall,
+
+    type: VideoFeedType,
+
+    // maxHeight style attribute for the video element
+    maxHeight?: number,
+
+    // a callback which is called when the video element is resized
+    // due to a change in video metadata
+    onResize?: (e: Event) => void,
+}
+
+export default class VideoFeed extends React.Component<IProps> {
+    private vid = createRef<HTMLVideoElement>();
+
+    componentDidMount() {
+        this.vid.current.addEventListener('resize', this.onResize);
+        if (this.props.type === VideoFeedType.Local) {
+            this.props.call.setLocalVideoElement(this.vid.current);
+        } else {
+            this.props.call.setRemoteVideoElement(this.vid.current);
+        }
+    }
+
+    componentWillUnmount() {
+        this.vid.current.removeEventListener('resize', this.onResize);
+    }
+
+    onResize = (e) => {
+        if (this.props.onResize) {
+            this.props.onResize(e);
+        }
+    };
+
+    render() {
+        const videoClasses = {
+            mx_VideoFeed: true,
+            mx_VideoFeed_local: this.props.type === VideoFeedType.Local,
+            mx_VideoFeed_remote: this.props.type === VideoFeedType.Remote,
+            mx_VideoFeed_mirror: (
+                this.props.type === VideoFeedType.Local &&
+                SettingsStore.getValue('VideoView.flipVideoHorizontally')
+            ),
+        };
+
+        let videoStyle = {};
+        if (this.props.maxHeight) videoStyle = { maxHeight: this.props.maxHeight };
+
+        return <div className={classnames(videoClasses)}>
+            <video ref={this.vid} style={videoStyle}></video>
+        </div>;
+    }
+}
+