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
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
Upgrade to Ruby 2.1.4
Merge pull request #448 from alphagov/upgrade_to_ruby_2-1-4

Upgrade to Ruby 2.1.4
Bump admin gem to 1.1.7

* Fix lead styles when using the `text-muted` class
Merge pull request #450 from alphagov/fix-lead-styles

Fix lead styles when using the `text-muted` class
Remove path_hash everywhere but the DB

* Add `canonical_path` (with index) to host_paths to compensate
* Change NOT NULL path_hash fields to NULLable (or we'd still have to calculate them)
* Cope with any possible rollback in the migration's `down`
Merge pull request #449 from alphagov/remove-path-hash-from-transition-entirely

Remove path_hash everywhere but the DB
Revert "Remove path_hash everywhere but the DB"

This reverts commit 08b1a93cbaa74b455ea50067921aa10dcc05de79.

We need to do this because we've spotted that we've put a unique
call in the wrong place, leaving an index non-unique. This could
be reverted in a second migration, but we also spotted that
we haven't dealt with the plethora of NULL in the new canonical_path
column that a new UNIQUE would require. At time of commit, this has
only reached preview, so we'll massage the indexes there.
Merge pull request #451 from alphagov/temporarily-revert-path-hash-removal

Revert "Remove path_hash everywhere but the DB"