This caused subtle bugs when ALTER TABLE with search_title was
called. It is a wrong practice to define an index immutable when in
fact it's not. The count(id) part makes it mutable in fact.

This reverts commit 29ee1972.

It introduces a bug, it should not be merged.

However, the code doesn't work currently.

This has couple of drawbacks:
- we use postgres-simple and a raw sql query, opaleye would be better
I guess
- there is a hack for empty query, this should be replaced by
removing the WHERE clause in this case

This is to match the { tag | value } on the frontend.