On room creation, A list of initial state events may be provided that
should be submitted to the room after creation. These events
should be treated as any other state event and submitted to
the same checks.
With this change, state events submitted on room creation will be
submitted through the same helper as those through the usual endpoint.
The submission helper is moved to the timeline service to make it
available everywhere. This will be useful for implementing MSC4140
(issue #903).
