Remove the advice to run the import:hits_mappings_relations rake task

- In writing some instructions on how to generate mappings from hits for
  sites in preview and production, I wondered about the
  `import:hits_mappings_relations` rake task and its purpose. Following
  feedback from Jenny, it seems like the purpose of this rake task is
  fulfilled by
  https://github.com/alphagov/transition/blob/master/app/models/mapping.rb#L42.
  This means that the advice at the end of `import:mappings_from_host_paths`
  to run it can be removed.
- The rake task itself is being kept and its description updated to
  "Dev only" because it's still useful in dev.
Order the hosts API output based on the hostname, A-Z
Merge pull request #442 from alphagov/correctly_order_hosts_api

Order the hosts API output based on the hostname, A-Z
Merge pull request #434 from alphagov/remove_import_hits_mappings_relations_rake_task

Remove the advice to run the import:hits_mappings_relations rake task (when importing mappings)
Be consistent with production: don't set synchronous_commit in dev

- This was used for Postgres in dev for some reasons, but isn't set in
  production, and we should have no disparity between production and dev
  anymore. Remove it from the test database config as well for good
  measure.
Remove the local_infile setting as it is MySQL specific
Remove the config block for the production database

- The whole file is overwritten on deploy, so it's confusing for those
  of us looking at the checked out database config and it not being the
  same as production.
Merge pull request #443 from alphagov/remove_synchronus_commit_setting_from_dev

Be consistent with production: don't set synchronous_commit in dev
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