Invite users to existing rooms (#441)

Invite users to existing rooms

Scope:

- Allow inviting from the room detail screen and the member list
- Invite option is only shown if the user has the correct power level
- Search flow the same as creating a new room, allowing multi-select
- Existing room members/invitees are disabled with a custom caption
- Sending is asynchronous, an error dialog will appear wherever the
  user is if necessary

Closes #245

changelog.d/245.feature

@@ -0,0 +1 @@
+[Create and join rooms] Add ability to invite users to existing rooms