Simpler mappings logic, and add tests
Add export:redirector_mappings Rake task

This dumps out the redirector mappings to a known location on disk. The
CSV can then be automatically downloaded by the Redirector build script.
Use export:redirector_mappings in the cron job

This replaces the old and now removed script.

Additionally clean up the now unused job_type.
Merge pull request #802 from alphagov/fix-attachment-source-mappings

Fix attachment source mappings