Add a test for http_status being set from type on batches

Also add comments explaining why http_status is left the same if it
isn't supported.
Migrate http_status data to type in a migration

- Also support rolling back, to populate http_status from type
- Remove the rake task and its tests
- Also set type on batches as well as mappings
Tell PaperTrail to not track changes to http_status

We've temporarily got two fields storing this information, and we
don't want them both to show up on the history tab duplicating
the same change.
Use the type-specific mapping factories in all specs

These tests presumably date from before the existence of the :redirect
and :archived mappings factories. Using these factories makes the lines
shorter and removes some http_status -> type changes from the overall
diff.