Code reference archive pages need filter

[siobhan]

Lots of the archive pages have functions, hooks, methods, and classes all jumbled together. For example the <a href="​http://developer.wordpress.org/reference/since/3.0.0/">since archive pages</a>.

There should be a way for users to filter the result (as we have implements on the search results page). It could look something like this:

[keesiemeijer]

Add a filter form to taxonomy archives

[keesiemeijer]

Filter dropdown

[coffee2code]

In 2966:

developer.wordpress.org: Add a filter form to taxonomy archives.

Props keesiemeijer.
Fixes #601.