Add UX to communities/groups right panel to set admins

[benparsons]

Want

It's possible to have multiple admins for a community, and to add/remove admins. I would like a UX for this in the right panel.

This could be based on the existing groups right panel + the admin UX from the rooms right panel.

Notes

There will be a potential problem in the form of:

• a is admin
• a makes b admin
• b removes adminship from a
• a writes to support@matrix.org to complain

I think this is an acceptable risk given that it's currently impossible to manage communities through riot.

Also: this is purely an MVP stopgap until the expected communities overhaul. This is, I think, the minimum work needed to really make communites useful in the short-term.

[t3chguy]

Is there a matching Synapse/spec issue for Groups to support this?

[turt2live]

This is not possible currently. Duplicate of #5240