feat(screenshare): add screen bitrate config in bbb-html5

...on-html5/imports/api/screenshare/client/bridge/kurento.js

...on-html5/imports/api/screenshare/client/bridge/service.js

.../imports/ui/services/bbb-webrtc-sfu/screenshare-broker.js

bigbluebutton-html5/private/config/settings.yml