add feat Breakout Room Messaging/Broadcast message

[Bloodiko]

This PR adds lib functionality to improve Text Messaging in the following ways:

• onedirectional "Info"-messaging to all rooms (including main room for backlog reasons?) AKA Broadcast

• onedirectional "Info"-messaging to a single (Breakout) Room

• bidirectional "Private Messaging" across all rooms

Keep in mind that broadcasting messages should only be allowed for moderators maybe and only work within the same conference, not across conferences.

Current behaviour: Message to any room, except the joined room returns an error message from the websocket. Message to any participant outside the own room returns nothing.

Proposed behaviour: Message to any room within the boundaries of the conference by a moderator should be allowed, as well as broadcasting a message to all rooms in the conference. Private messages from any participant to another participant in a different room should be allowed.

Please tell me what you think about this. I am not experienced enough to create a PR on the backend, so that side will be missing entirely.

[jitsi-jenkins]

Hi, thanks for your contribution! If you haven't already done so, could you please make sure you sign our CLA (https://jitsi.org/icla for individuals and https://jitsi.org/ccla for corporations)? We would unfortunately be unable to merge your patch unless we have that piece :(.