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
