Invite to chat facilities [tracking]

[gnunicorn]

Currently we can create chats, but then can't invite anyone from the acter UI. This is a tracking issue as we have multiple aspects to take care of/for here. In order of easiness to implement:

• [x] ability to invite users to a chat from within a chat -- should reuse the existing invite-to-space-widgets here, should be fairly straight forward to implement
• [x] list of users of a chat (needs some designs) -- can probably also be analogous to the space-members-widget, too
• [x] change power-levels of users in a chat (as above)
• [ ] during the creation process:
  • [ ] asking who should be invited to a space with options to invite everyone from the space this is part of
  • [ ] allowing to configure join-rules (invited, public-for-space, knock, etc)