Support for per-circle owners/administrators

[decentral1se]

As an addition to #58 with focus on circle admins: it came up in the general chat that it would be nice to be able to able to customise the avatar of the main circle. There is currently a work-around documented in https://gist.github.com/horazont/1e80c64315dc67dc091ad037914cafd1. Maybe there is other stuff to request on this ticket idk.

[mwild1]

As discussed at #58 (comment) there are a number of use-cases warranting the ability to have some control over circle MUCs.

We need to be careful that the state of the MUC cannot be overridden via XMPP such that it becomes out of sync with the internal circle state. One way to ensure this is to have the MUC be the source of truth for the circle state. We would also have to block or figure out how to handle users on remote servers being added.

For the app UI, a circle owner/admin should be able to invite new users to the server+circle if they have permission to invite users at all.

Note: by "owner/admin" I am not proposing two roles, I think one is enough. But we'd need to figure out the UI terminology and the specific XEP-0045 affiliation to use.