Make mjolnir able to join knock only rooms/banlists

**Is your feature request related to a problem? Please describe.**
We have a banlist, that shouldn't be public. It would be nice if mjolnirs could just knock on that room instead and we accept the knock for bots we trust.

**Describe the solution you'd like**
When a banlist is not public, but has knock or knock_restricted join rules, the bot should try knocking and then accept the invite after that.

**Describe alternatives you've considered**
I could manually invite the bots, but that is a lot more effort. I could also require admins to join the banlist room first and then have them invite their bot, but that sounds annoying too.

No files or tests are named in the issue. Start by locating the handling for non-public banlists and Matrix rooms using knock or knock_restricted join rules; done means the bot knocks on the banlist room and accepts the invite after the knock is approved.

Written by the indexing model from the issue text.