feat(api) allow filmstrip autohide to be configurable

config.js

     //     'toolbar.noisyAudioInputTitle', // shown when noise is detected for the current microphone
     //     'toolbar.talkWhileMutedPopup', // shown when user tries to speak while muted
     //     'transcribing.failedToStart' // shown when transcribing fails to start
-    // ]
+    // ],
+
+    // Automatically hides the filmstrip when screen width is under a certain threshold
+    autohideFilmstrip: true,
 
     // Allow all above example options to include a trailing comma and
     // prevent fear when commenting out the last value.

react/features/base/config/configWhitelist.js

     'analytics.disabled',
     'audioLevelsInterval',
     'audioQuality',
+    'autohideFilmstrip',
     'apiLogLevels',
     'avgRtpStatsN',
     'backgroundAlpha',

react/features/filmstrip/subscriber.web.js

 StateListenerRegistry.register(
     /* selector */ state => state['features/base/responsive-ui'].clientWidth < ASPECT_RATIO_BREAKPOINT,
     /* listener */ (widthBelowThreshold, store) => {
-        store.dispatch(setFilmstripVisible(!widthBelowThreshold));
+        const state = store.getState();
+        const { autohideFilmstrip } = state['features/base/config'];
+
+        if (autohideFilmstrip) {
+            store.dispatch(setFilmstripVisible(!widthBelowThreshold));
+        }
     });
 
 /**