Simplify title on user accounts page

* Match link in header
Switch suspended column to a status one

* See whether users are active, suspended, invited or if their
passphrase has expired
Allow filtering by user status
Split filters and create user buttons in layout

Give more space to filters before adding new filter
Add status filter to user list
Whitespace: User index indentation
Add filtered user counts above users table
Move user filters into partial
Move user filter methods to separate helper
Use the same methods for role and status filters

Abstract out the filter type to make management of filters easier and
simplify markup.
Port filter styles from Transition app

Bring across white filter styles from Transition. These should
eventually live in the admin template.

* Introduce a nag-compact style which has less padding
* Highlight active filters
* Introduce a remove filter button
* Reduce size of chevron in filter
* Add ‘pill’ styles for text only items in a nav-pill container
Add a 'remove all filters' link

Quickly return to the original list of users
Tweak alignment of page titles

When titles are floated or within grids/containers, the margins
collapse differently, which pushed the title further down the page than
it ought to be.
Merge pull request #296 from alphagov/user-account-counts

Allow filtering of user list by user status