Add a way to gauge the inviting user

I recieved a few invites from unknown users, and I had no idea who they were/came from. I could check their profile to see if we had any rooms in common, but there was no way to actually check their profile!

This makes it possible to open their profile by clicking their avatar (like elsewhere) but also adds a button to make it even more obvious.

I also fixed an issue where you couldn't view the profile for an existing DM (or invites) without messaging first.