[bug 761573] Add esunified waffle flag

This waffle flag will waffle between the ES bucketed search results
view that we have now and the new ES unified search results view of
the glorious future.

* off: bucketed search results (i.e. wiki, then questions, then forums)
* on: unified search results (i.e. everything sorted by score)

migrations/147-elasticsearch-unified-waffle.sql

@@ -0,0 +1,2 @@
+INSERT INTO waffle_flag (name, superusers, staff, authenticated, rollout, created, modified)
+VALUES ('esunified', FALSE, FALSE, FALSE, FALSE, NOW(), NOW());