join group by invite link command

[maltee1]

Depends on #501 to create the room when the join request is accepted. There'll be an incompatibility though, this one needs to be rebased after merging #501.

I was also thinking that it might be nice to generate a matrix.to link via which the room can be joined (using knocking when admin approval is required), but this works for now, so just let me know what you think and I might implement it later.