Very basic Jitsi integration

Replaces the conference calling by adding a Jitsi widget. No support
for continuing the call on room change currently, so hidden behind
a feature flag. Existing behaviour is preserved by default.

src/settings/Settings.js

@@ -1,5 +1,6 @@
 /*
 Copyright 2017 Travis Ralston
+Copyright 2018 New Vector Ltd
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -88,6 +89,12 @@ export const SETTINGS = {
         supportedLevels: LEVELS_FEATURE,
         default: false,
     },
+    "feature_jitsi": {
+        isFeature: true,
+        displayName: _td("Jitsi Conference Calling"),
+        supportedLevels: LEVELS_FEATURE,
+        default: false,
+    },
     "MessageComposerInput.dontSuggestEmoji": {
         supportedLevels: LEVELS_ACCOUNT_SETTINGS,
         displayName: _td('Disable Emoji suggestions while typing'),