Use govuk admin template

This should keep the search-admin more consistent with other admin apps.

Story: https://www.pivotaltracker.com/story/show/73318370
It looks like the `javascript_include_tag` is being included twice, once in :head, and once here (although this and the script block below aren't in a `content_for` block, so I _think_ will render at the top of the page. Perhaps they should both be in `body_end` at the foot of the page? (Sorry I missed the initial review on this)