Scope on User to filter by role

https://www.agileplannerapp.com/boards/173808/cards/4223

working on a filter by role functionality on the users
index page. these are related model changes to enable
that.
Controller changes to filter users by role

also, a few improvements to tests related to filtering.
Adding link to users index for filtering by role

also, fixed the 'Clear' button which was not clearing
the query params as expected.
Integration test for filter by role
Styling for filter panel
Update title & menu label to indicate role filter
Non-JS version of filter by role
Display list of roles sorted by level
Merge pull request #243 from alphagov/filter-by-role

Filter by role