Make proposed breakouts individually deleteable

[kitsandkats]

Right now, admins can only clear the entire set of breakout rooms; it would be nice to have rooms that are deletable/hide-able individually for easier clean-up and moderation.

[fields37]

@kitsandkats @dirkcuys

It might also be helpful to have the ability to delete all the empty breakouts, or ones with only one or two people, to help encourage consolidation.