Basic presentation of organisation search filter

Not wired up to get real data, nor does the filter have any effect.
Organisation filter retains tab param on submit
Retain the organisation selection on filter
Pass the organisation filter to Rummager
Fetch the organisations list from Rummager
Constrain the width of the organisation dropdown

Some of the organisations have very long names. Technique approved by
@tombaromba
Sort organisations in the filter
Retain organisation filter on a new search
Always show all search tabs

If you run a search that returns no Government results, or no government
results with the current organisation filter, the tab would be hidden.

This is problematic for several reasons:
* if you were filtering down results in the government tab, it's suddenly
  disappeared. If you want to change the organisation filter, you can't.
* it appears that there are no results, even though there may be, if there was
  no organisation set. You can't see that you're filtering to an organisation,
  nor which organisation it is.
Hide the organisation filter behind a feature flag

We need to play with the UX and design, and user test it before we're confident
enough to turn it on by default.
Overhaul the tab selection logic

Since all tabs were enabled if there were any results (in https://github.com/alphagov/frontend/commit/98e46d50b52a80ffb4c77d6b568c6b9d318af41d )
the behaviour of tab selection has been suboptimal. If you had specified a tab,
it would be selected. However if you hadn't, it would default to the first tab,
even if that was empty and another tab had results.

The active tab is now decided in this order:
  1. user-selected tab (supplied in the ``tab`` querystring parameter)
  2. the first tab with results
  3. the first tab
Display "0 results" in tabs without results

This makes it clearer when the tab is empty.
Merge remote branch 'origin/master' into department_search_filtering

Conflicts:
	test/functional/search_controller_test.rb
Show the tabs if there is a filter in place

We want to show the tabs if there's a filter in place
because there might be results with the filter turned off, but you can't
do that if the filter-form/tabs aren't displayed.
Consistent whitespace for no results message

Courtesy of @markhurrell
Merge pull request #346 from alphagov/department_search_filtering

Government organisation search filtering