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/search.py

@@ -347,9 +347,6 @@ class ConductRepositorySearch(ApiResource):
   def get(self, parsed_args):
     """ Get a list of apps and repositories that match the specified query. """
     query = parsed_args['query']
-    if not query:
-      return {'results': []}
-
     page = min(max(1, parsed_args['page']), 10)
     offset = (page - 1) * MAX_PER_PAGE
     limit = offset + MAX_PER_PAGE + 1