While this means we need an additional query for initial lookup, it makes the *filtering* query (which is the heavy part) require far fewer joins, thus making it more efficient.
Also adds a new unit test to verify that our filter filters to the correct set of repositories.
Allows for exploration of all visible repositories, in paginated form.
This change also fixes the layout of the header on different viewport sizes to be consistently a single line in height.
Fixes https://jira.coreos.com/browse/QS-63
