Tidy up document filters

- tidy up document list spacing
- unify document filter styles (all select boxes are dropdowns, all
checkboxes are left aligned)
- change filter title typography from large/regular to small/bold
- align documents list on filter pages with the gov.uk nav
- remove multi-select filters
- add non-breaking spaces to filter description to make the
word-wrapping nicer
Align filters with govuk search box and bugfix

- align filters with govuk search box
- re-apply absolute positioning to date-range visually-hidden checkbox
Make select boxes return array like values

Even though these are no longer multi-select inputs they should still
behave like they are. There is talk of adding a more advanced form to
create a filter in the future and that will probably make use of
multi-selects.
Merge pull request #446 from alphagov/simplify-document-filter

Tidy up document filters