campaigns: Introduce HTTP endpoints for changeset syncing
Created by: ryanslade
These RPC methods are intended to be used from the frontend when we add the ability to force sync a changeset.
This is the second round of changes. The first removed the syncer from the frontend but after considering the extra complexity involved we've moved back. See commit comments for details.
The next PR will introduce a proper scheduler and make the enqueuing of changesets truly async.