Add a data migration to trim excessive slugs

We introduced a limit on slug length in ca28979, however documents
created before this time don't conform to this validation, which breaks
the crawler.

This data migration cleans up the old slugs for published editions by
updating their slugs to the new maximum length and redirecting the old
URLs to the new URLs.
Commit the redirects to the router
Switch tab and masthead colours

* Try and match other admin apps a little more, use a darker
background, and lighten the tabs.
Merge pull request #1650 from alphagov/fix-slugs-that-are-too-long

Add a data migration to trim excessive slugs
Re-order Department and Topic filters

Re-ordering the department and topic filters so they are consistent with the Statistics Published tab and Publications filters; Topic followed by Department.
Merge pull request #1653 from alphagov/reorder-department-topic-filter

Re-order Department and Topic filters
Add crown icon and app name to header
* Match other admin applications
* Clarify name of application
Match other apps environment indicators
Indent crown for better alignment with tabs
Introduce coloured environment bar

* Match other admin apps with a 5px coloured bar stretching across the
header
* Because of tabs this is at the top of the masthead rather than the
bottom
Merge pull request #1651 from alphagov/dark-header

Blacken Whitehall admin header