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)
Validate 2048 length in path/new_url

* Companion to https://github.com/alphagov/transition/pull/444
* That PR unblocked innovateuk, this commit makes sure that the UX matches the
  DB limits
Merge pull request #447 from alphagov/length-validation-should-match-columns

Validate 2048 length in path/new_url
Merge pull request #446 from alphagov/path-hash-removal-pt-1

Add UNIQUE (site_id, path) index to mappings