3632 Filter collections by type

http://code.google.com/p/otwarchive/issues/detail?id=3632

You can only choose one type 'cause that's how all our filters work at the moment.

This also moves the sorting options (title and date) out of the navigation and into the filters, so you can do two things at once. I went with select menus rather than radios to maintain consistency with other areas of the site (sort options on other filters and the work search form are done as select menus). This incorporates the fix for Issue 3791 (Logged out users should be able to see open challenges) found in pull request #1374 and changes the HTML structure of the collection navigation, since it now consists of only one ul.