Switch tab and masthead colours

* Try and match other admin apps a little more, use a darker
background, and lighten the tabs.
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
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