Add IE9-specific stylesheet
Remove fancy select styling for IE <= 9 on stats announcement filter

The select dropdown arrows can't be hidden without some serious hackery
in IE9 and can't be hidden at all in IE <= 8. It's better to strip back
any fancy styling for IE.
Use same styling for publication & stats announcement filters

Previously, the styling for the publication filter and stats announcement
filter were using two different versions of similar css. This modifies the
publications filter to use the more generic filter-form-block css helper.
Merge pull request #1577 from alphagov/fix-selects-in-ie

Fix selects in ie

Story: https://www.pivotaltracker.com/story/show/73412800

Previously, select boxes on the frontend filters were displaying with two down arrows - one from the background image, and one browser-rendered dropdown arrow. This PR strips out the more complicated select styling for IE version <= 9.

I needed to introduce an IE9 stylesheet to achieve this.

This PR also rationalises the styling between the publications filter and the stats announcement filter.