Rewrite query to fetch all story tags first and only then filter by tag set
Rewrite query to fetch all story tags first and only then filter by tag set

Before:

https://explain.depesz.com/s/6SAr

After, FIXME:

https://explain.depesz.com/s/x1Hx

Fixes #360.

berkmancenter/mediacloud,berkmancenter/mediacloud,berkmancenter/mediacloud,berkmancenter/mediacloud,berkmancenter/mediacloud