[Do not merge yet] Unify whitehall/govuk_template header bar

**Not to be merged until https://github.com/alphagov/static/pull/404 has been merged/deployed**, but I wanted to get review done so this is ready to go.

* Remove custom whitehall bar
* Unhide the global template bar and style it like the whitehall one
* Remove custom dept styling CSS, which wasn't actually be applied anywhere

The `#global-header-bar` colour override feels a little brittle to me. Using shared template/html is an improvement, but it introduces a dependency on knowing/staying in sync with the markup in the template. 

An approach i've used in the past for this kind of thing is providing a SASS mixin that encapsulates knowledge of the common markup. This has it's own issues with how that mixin gets made available to the app, and how versions are managed for that. Possibly a good one to discuss in the next FE meeting.