Room power levels for community members

**Feature Request:**

It would be useful to be able to set power levels for community members for a room. So if a user is a member of a community, I should be able to set all community members for a room to have moderator level permissions for a room, or give community members elevated power levels for a room.

Something like setting a rooms m.room.power_levels but with +community_name@homeserver.url groupId you're setting power levels for?

Start by reading the existing Matrix room power-level specification and the community membership model. Determine how room permissions should apply to all members of a community, including the proposed community identifier format; done means the protocol behavior and edge cases are specified clearly enough for implementations to follow.

Written by the indexing model from the issue text.