Improve admin editions filter

https://www.pivotaltracker.com/story/show/61476834

Although the functionality is there, I'm still not quite happy with the code.  Due to time pressure from code-freeze, I'm putting this up now, but will make a further pull request to address the following:

* editions/index.js script is a little messy - among other things I'm not happy with how functions need proxying.  I intend to introduce a javascript helper to wrap prototype methods in a proxy function.
* The search filters view could really use a bit of DRYing and a bit of markup improvement (use fieldsets where appropriate, etc.).