Implements Recent Searches

This pull request adds a recent searches dropdown list to the listing pages in the admin ui.

![screenshot](https://dl.dropbox.com/s/m2h4f7l27dl848b/Screenshot%202014-05-29%2015.50.34.png)

If there are no recent searches, the dropdown is hidden.

It uses the new keystonejs/queryfilter library to use the same filter code on both the backend and frontend.