Peace and harmony, in all the URLs

Make bulk add and import cope with `new_url`s up to 2048 characters in length.
mySQL used to silently truncate long `new_url`s, Postgres is more honest.
Harmonise path lengths

* `mappings.path` gets a boost to 2048 from 1024
Drop mappings_staging

* Had a `path` in it that we would have wanted to change length for
* If we hadn't stopped using the table altogether in
  d014d4f2c133609c963a9db8875f50f8745b22ab
Merge pull request #444 from alphagov/harmony-in-all-the-url-lengths

Peace and harmony, in all the URLs