stop external video if presenter changed

bigbluebutton-html5/imports/api/external-videos/index.js

.../api/external-videos/server/handlers/stopExternalVideo.js

...ternal-videos/server/methods/stopWatchingExternalVideo.js

...eos/server/methods/stopWatchingExternalVideoSystemCall.js

...rts/api/screenshare/server/handlers/screenshareStarted.js

...ml5/imports/api/users/server/modifiers/changePresenter.js

...on-html5/imports/api/users/server/modifiers/removeUser.js