Add Explore tab and query-less searching

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

endpoints/api/test/test_search.py

@@ -9,6 +9,7 @@ from endpoints.test.shared import client_with_identity
 from test.fixtures import *
 
 @pytest.mark.parametrize('query', [
+  (''),
   ('simple'),
   ('public'),
   ('repository'),