Simplifying the test for whether or not a user and slug combo exists.

Thanks to tchernobog for catching this (it was breaking on postgres)
and Elrond for the suggestion on how to fix it.

This commit sponsored by Caleb Cooper.  Thanks Caleb!

mediagoblin/db/migrations.py

@@ -215,12 +215,10 @@ def mediaentry_new_slug_era(db):
     import uuid
 
     def slug_and_user_combo_exists(slug, uploader):
-        # Technically returns the number of entries with this slug and user
-        # that already exist
         return db.execute(
             media_table.select(
                 and_(media_table.c.uploader==uploader,
-                     media_table.c.slug==slug)).count()).first().tbl_row_count
+                     media_table.c.slug==slug))).first() is not None
 
     def append_garbage_till_unique(row, new_slug):
         """