Allow callback functions for creating jobs

Allow callback functions for creating jobs

$wgJobClasses can now specify a factory function for creating a job,
instead of a class to be instantiated directly. This makes it possible
to inject services in a job constructor, and register a factory function
that calls the constructor with default services.

This follows Ieb85493a7765 and Ia2107dc5af78, which introduced factory
functions for API modules and special pages.

Change-Id: I0461e59da2a8fa6681e3b1fcdfc38bfed7f3ac32