feat(external_api) allow shortcuts to be disabled

conference.js

     _onConferenceJoined() {
         APP.UI.initConference();
 
-        APP.keyboardshortcut.init();
+        if (!config.disableShortcuts) {
+            APP.keyboardshortcut.init();
+        }
 
         APP.store.dispatch(conferenceJoined(room));
     },

config.js

     // will be joined when no room is specified.
     enableWelcomePage: true,
 
+    // Disable app shortcuts that are registered upon joining a conference
+    // disableShortcuts: false,
+
     // Disable initial browser getUserMedia requests.
     // This is useful for scenarios where users might want to start a conference for screensharing only
     // disableInitialGUM: false,

react/features/base/config/configWhitelist.js

     'disableRemoteControl',
     'disableRemoteMute',
     'disableRtx',
+    'disableShortcuts',
     'disableSimulcast',
     'disableThirdPartyRequests',
     'disableTileView',