Support for knocks
This PR adds the ability to add friends to circles, groups, galleries, and your "people" connections, by showing them a QR code.
The friend scans the QR code to extract your roomId and then sends you a knock.
The app shows when users are knocking on each kind of room and gives you the option to accept (invite) or reject (kick) their request.
From there, everything proceeds as normal. The friend gets an invitation and it is handled like normal. This last step with the invitation is weird, but that's how Matrix works.
The next step for all this stuff will be to add support for "Invite by link" instead of using the QR codes. This is more friendly for users who are on mobile devices but who are not physically in the same space.
Closes #134 (closed)