Update filter heading

* Also cleanup margins on filter heading
Make filter placeholders match style guide

* eg not e.g.
(https://www.gov.uk/design-principles/style-guide/style-points)
Use Bootstrap button styles for filter submit

* Make the action more prominent and consistent with the rest of the
application, also separates it a bit from the reset link below
Correctly pluralise published document counts

* eg 1 published document, 2 published documents
* Remove `strong` tags from around numbers
Use smart quotes in document filter page titles

* http://smartquotesforsmartpeople.com/
Give featured list filters more room

* Match document list filters by using the same sized grid elements
* Remove custom span2 styling
Emphasise filtered table header

* Use table-header styles brought across from govuk_admin_template
* Make feature filter and document filter table styles match
Add heading to feature filter search results
Add clearer 'no documents found' messaging

* Use the no content styles brought across from govuk_admin_template
Bring back filter input border radius

* Match other form elements within app
Add line breaks between filter option selectors

* For clarity