Switch to using mustache templates for filter text

First pass at switching to using mustache to render the filter text. I
have tried to make the whole thing work without JavaScript. For this I
have added some methods to create urls without the filter bit in it.

I have also added some helper methods to get the data ready for
mustache.
Add keywords to filter sentance and enable removal

Added joining phrases to mustache objects so sentances can be
made.

Added the rendered sentance to all filter pages not just the
announcments and publications lists.

Styled to match the new markup.

Made javascript handle removing keywords.
Use published gem
Update tests and bump gem so it could happen
Merge pull request #176 from alphagov/shared-mustache

Shared mustache and keywords on document filter sentence