RIP out mongo

Since sqlalchemy is providing our database abstraction and we have
moved away from Mongo as the underlying database, it is now time to
simplify things and rip out mongo. This provides the bulk of the
changes, and can stand on its own. There are some followup tasks
that can be done, such as removing now unneeded abstraction layers,
e.g. db.sql.fake.py

mediagoblin/listings/views.py

@@ -36,10 +36,6 @@ def _get_tag_name_from_entries(media_entries, tag_slug):
                 tag_name = tag['name']
                 break
         break
-    # TODO: Remove after SQL-switch, it's mongo specific
-    if hasattr(media_entries, "rewind"):
-        media_entries.rewind()
-
     return tag_name