Support for HSes (or ASes) to participate in a room irrespective of whether members are present (SPEC-160)

[matrixbot]

Currently, a room can only be replicated on a HS if that HS has a user present in the room. This introduces some potential limitations:

• You can't deliberately replicate a room over a cluster of HSes for HA
• The HS (or AS) can't participate in a room as an admin superuser as suggested in https://github.com/matrix-org/matrix-doc/issues/498, to ensure there's always a 'god' user present in case of disaster.
• You can't have invisible AS users
• ...probably others i've forgotten.

(Imported from https://matrix.org/jira/browse/SPEC-160)

(Reported by @ara4n)

[matrixbot]

Jira watchers: @erikjohnston @ara4n

[matrixbot]

Other use cases:

• Allow HSes to have aliases for rooms even if all its users have left.
• Allow rooms to be kept alive if all users leave.

-- @erikjohnston

[matrixbot]

One way to implement this is by allowing generic entities to send events rather than just users; where entity here is probably just users and servers.

-- @erikjohnston

[richvdh]

seems to be a duplicate of matrix-org/matrix-spec#137