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
Delete redundant feature flag code

Both these features have been live for a long time now and no longer
require feature-flags. The tests for Whitehall.government_search_index
are questionable, but I've left them here for safety.
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
Make it clearer what the rummager rake tasks do

It wasn't clear that these tasks index more than just "documents". In
fact they either index detailed guides, or everything else that is
searchable.
Rename misleading methods

At some point in the distant past, Whitehall.government_edition_classes
did actually return only Edition classes. Now, it returns all classes
that are searchable and should be indexed in the "government" index,
i.e. all searchable classes except DetailedGuide. I've renamed these
methods to avoid confusion in the future.
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
Merge pull request #1654 from alphagov/remove-old-feature-flag-code

Delete redundant feature flag code
Don't override passed in STATIC_DEV env var in startup.sh

Instead, conditionally set it to a default.
Merge pull request #1657 from alphagov/tweak-startup-script

Don't override passed in STATIC_DEV env var in startup.sh
Merge pull request #1656 from alphagov/rewrite-code-for-clarity

Rewrite code for clarity
Rename feature file to be more generic
Failing feature for cancelling stats announcement