Add search fields to admin list displays.

To make it easier to find rows quickly, add a search field to the
admin list display.
Show data group and type on admin list display for datasets.

To make it easier to browse datasets in the admin interface, add
data group and type to the list display.

These columns are sortable, so it's easy to find all the datasets
related to a particular group or type.
Merge pull request #107 from alphagov/add-search-and-browse-to-admin-list-display

Add search and browse to admin list display