Improve admin editions filter

Implements: https://www.pivotaltracker.com/story/show/61476834

Primarily, this PR modifies the admin editions filter to retrieve results via an XHR request instead of simply reloading the page.  It also modifies the filter form to be a single form instead of an individual form for each filter.

Part of it's implementation involved introducing a 'proxifier' - an object to quietly handle the issue of needing to proxy up function calls in eventy JS.  More info about that is in the style guide: https://github.com/alphagov/whitehall/blob/0452a2435bd82a14e8948c7b0a2fb0c0749bd883/STYLEGUIDE.md#proxying-function-when-using-the-prototype--constructor-pattern