The most common thing that we do with fab! is:
fab!(:thing) { Fabricate(:thing) }
This commit adds a shorthand for this which is just simply:
fab!(:thing)
i.e. If you omit the block, then, by default, you'll get a `Fabricate`d object using the fabricator of the same name.
Currently, the logic for creating a new chat message is scattered
between a controller and an “old” service.
This patch address this issue by creating a new service (using the “new”
sevice object system) encapsulating all the necessary logic.
(authorization, publishing events, etc.)
