Add UNIQUE (site_id, path) index to mappings

* Part one of removing path_hash. See
  https://www.pivotaltracker.com/story/show/74294416
* This will incur a small write performance hit when creating
  large batches of mappings (unmeasured)
Merge pull request #446 from alphagov/path-hash-removal-pt-1

Add UNIQUE (site_id, path) index to mappings