feat(external_api) allow initial gUM requests to be disabled

conference.js

      */
     createInitialLocalTracks(options = {}) {
         const errors = {};
-        const initialDevices = [ 'audio' ];
-        const requestedAudio = true;
+        const initialDevices = config.disableInitialGUM ? [] : [ 'audio' ];
+        const requestedAudio = !config.disableInitialGUM;
         let requestedVideo = false;
 
         // Always get a handle on the audio input device so that we have statistics even if the user joins the
             this.muteAudio(true, true);
         }
 
-        if (!options.startWithVideoMuted
+        if (!config.disableInitialGUM
+                && !options.startWithVideoMuted
                 && !options.startAudioOnly
                 && !options.startScreenSharing) {
             initialDevices.push('video');
             requestedVideo = true;
         }
 
-        JitsiMeetJS.mediaDevices.addEventListener(
-            JitsiMediaDevicesEvents.PERMISSION_PROMPT_IS_SHOWN,
-            browserName =>
-                APP.store.dispatch(
-                    mediaPermissionPromptVisibilityChanged(true, browserName))
-        );
+        if (!config.disableInitialGUM) {
+            JitsiMeetJS.mediaDevices.addEventListener(
+                JitsiMediaDevicesEvents.PERMISSION_PROMPT_IS_SHOWN,
+                browserName =>
+                    APP.store.dispatch(
+                        mediaPermissionPromptVisibilityChanged(true, browserName))
+            );
+        }
 
         let tryCreateLocalTracks;
 

config.js

     // will be joined when no room is specified.
     enableWelcomePage: true,
 
+    // Disable initial browser getUserMedia requests.
+    // This is useful for scenarios where users might want to start a conference for screensharing only
+    // disableInitialGUM: false,
+
     // Enabling the close page will ignore the welcome page redirection when
     // a call is hangup.
     // enableClosePage: false,

react/features/base/config/configWhitelist.js

     'disableAP',
     'disableAudioLevels',
     'disableDeepLinking',
+    'disableInitialGUM',
     'disableH264',
     'disableHPF',
     'disableInviteFunctions',