Move as_a_user into Transition::History

...from Whitehall mappings CSV import
Use as_a_user in 2 tests for precision

This is clearer than setting the user and then making changes to
mappings in setup.
Remove a redundant mapping factory as_user use

We only need to use this now when we care about the user; if we don't
care who it is then the factory will use a stub user for the whodunnit.