Lock room

[edemaine]

Staff ability to prevent people entering/leaving. Motivation:

• Delivering a conversation that ends in a pset solution. [Srini]
• Making it impossible for students to look at an old conversation that has an answer in it. [David]
• Meeting with students and talking about sensitive information (like Zoom waiting room during regday).

Blocked on #53.

[edemaine]

I implemented the inability to join a locked room through the Comingle interface. But security is tricky because we can't really distinguish users (except admins vs. non-admins), so we can't really tell whether a non-admin user was already in a room when it was locked — yet we need to allow such users to stay in the room (and maybe even reload the page and stay in the room). So it's possible to work around this by manually editing the URL to include the room hash, which is difficult to distinguish from reloading. But I've made it annoying, at least, to do so.