Consolidate styling for document filters

Although The Stats announcement filter shared a lot of
styling with the other document filters, it didn't share
all of it. This consolidates the styling of all the document
filters so they are all using the same CSS.
Strip superfluous markup from document filters

The document filter markup seems to have grown over
time. This removes most of the extra no-longer-needed
tags, classes and ids. This also brings the document
filter markup more inline with the stats announcement
markup.
Dry up some filter form markup
Merge pull request #1585 from alphagov/consolidate-document-filter-styling

Consolidate document filter styling