remember user choice regarding appver and platform facets in search (bug 729414)

The issue(s):

- when building the search form, we don't use request.session['search.appver'] (so empty value for the hidden input "id_appver")
- the js file media/impala/js/search.js will always set a default appver (and platform) even if the input is empty
- when updating (using pjax) the results and the url, we don't update the form's hidden inputs (appver, platform...)

This PR fixes those three issues by:

- using "any" instead of an empty string for the appver in the form
- only updating the hidden input fields (appver and platform) if there's no value
- also update the hidden input fields when clicking on the facets

There's also a few tests that are "fixed", that were previously failing (version_sidebar was using request.GET, but RequestFactory() doesn't have a GET querydict by default)