Fix robot user for Whitehall mappings import

This was finding and using the first user in the database, rather
than the first user with the robot user's email address.
Set whodunnit to user.name and set user_id

whodunnit is the user id as a string by default in Paper Trail, but
we set it to the user's name when versions are created through the
mappings controller and also set the version.user_id field. We need
to do this whenever versions are created outside the controller too
for consistency.
Update version data in mapping model spec

This is the obvious place to look for what a version's whodunnit
and user_id fields should be, so it was confusing to have tests
using PaperTrail's default data rather than what we are saving
from changes made via the controller. When we edit mappings in
imports or in any other way which does not use the mappings
controller, we need to make sure that the versioning data is
created consistently, so it's helpful to have that documented
accurately here.
Merge pull request #132 from alphagov/fix_whitehall_mappings_import_user

Fix robot user for Whitehall mappings import