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