feat(external_api): Add command for setting send/recv video quality

doc/api.md

 ```javascript
 api.executeCommand('receiverParticipantId', 'text');
 ```
+* **setVideoQuality** - Sets the send and receive video resolution. This command requires one argument - the resolution height to be set.
+```javascript
+api.executeCommand('setVideoQuality', 720);
+```
 
 You can also execute multiple commands using the `executeCommands` method:
 ```javascript

modules/API/API.js

 import { setE2EEKey } from '../../react/features/e2ee';
 import { invite } from '../../react/features/invite';
 import { toggleTileView } from '../../react/features/video-layout';
+import { setVideoQuality } from '../../react/features/video-quality';
 import { getJitsiMeetTransport } from '../transport';
 
 import { API_ID, ENDPOINT_TEXT_MESSAGE_NAME } from './constants';
         'e2ee-key': key => {
             logger.debug('Set E2EE key command received');
             APP.store.dispatch(setE2EEKey(key));
+        },
+        'set-video-quality': frameHeight => {
+            logger.debug('Set video quality command received');
+            sendAnalytics(createApiEvent('set.video.quality'));
+            APP.store.dispatch(setVideoQuality(frameHeight));
         }
     };
     transport.on('event', ({ data, name }) => {

modules/API/external/external_api.js

     password: 'password',
     sendEndpointTextMessage: 'send-endpoint-text-message',
     sendTones: 'send-tones',
+    setVideoQuality: 'set-video-quality',
     subject: 'subject',
     submitFeedback: 'submit-feedback',
     toggleAudio: 'toggle-audio',

react/features/video-quality/actions.js

+// @flow
+
+import { VIDEO_QUALITY_LEVELS } from '../base/conference';
+import logger from './logger';
+
+import type { Dispatch } from 'redux';
+
+/**
+ * Sets the maximum video size the local participant should send and receive from
+ * remote participants.
+ *
+ * @param {number} frameHeight - The user preferred max frame height for send and
+ * receive video.
+ * @returns {void}
+ */
+export function setVideoQuality(frameHeight: number) {
+    return (dispatch: Dispatch<any>, getState: Function) => {
+        const { conference, maxReceiverVideoQuality } = getState()['features/base/conference'];
+
+        if (frameHeight < VIDEO_QUALITY_LEVELS.LOW) {
+            logger.error(`Invalid frame height for video quality - ${frameHeight}`);
+
+            return;
+        }
+        conference.setReceiverVideoConstraint(Math.min(frameHeight, maxReceiverVideoQuality));
+        conference.setSenderVideoConstraint(Math.min(frameHeight, VIDEO_QUALITY_LEVELS.HIGH))
+            .catch(err => {
+                logger.error(`Set video quality command failed - ${err}`);
+            });
+    };
+}

react/features/video-quality/index.js

 export * from './components';
+export * from './actions';