Require approval of join requests

[TRManderson]

We can do this by either switching the codebase to another inviter and restyling, or adding some request storage thing (using postgres preferably - we've already got a postgres instance running on the server)

[CameronAavik]

relevant SO: https://stackoverflow.com/questions/41075562/is-approval-based-sign-ups-possible-with-slack