Make migration slightly more robust

For some reason, when I dropped and migrated my database from scratch, this
migration would fail because it was using a column definitions for the Site
table which changed some time ago. Defining local copies of the models didn't
fix the problem.
Merge pull request #467 from alphagov/make_migration_slightly_more_robust

Make migration slightly more robust
Drop path_hash

By now nothing is using it, not here or in Bouncer or in any part of the
DB (was being used, albeit harmlessly in
index_mappings_on_site_id_and_path_hash)
Merge pull request #468 from alphagov/goodbye-path-hash-we-knew-ye-too-bloody-well

Drop path_hash