Force users to listen only on some events

[matiasilva]

When a user or group creates a room, allow them to toggle the option to force everyone to join the room via "listen only mode". This should not apply to any group owners.

"listen only mode" is achievable by passing in a custom parameter to the "join" API call: https://docs.bigbluebutton.org/2.2/customize.html#application-parameters.

A new boolean field needs to be added the Room model and the logic needs to be handled in rooms.py on the update POST call. We also need to display the tickbox to the user in the manage.html file in rooms.