Bug889049 exploitability middleware needs pagination parameters

@selenamarie @rhelmer r?

This adds the ability to do things like: `curl http://localhost:8882/crashes/exploitability/page/3/batch/10`

I know the command `PYTHONPATH=. alembic -c config/alembic.ini upgrade 471c6efadde` creates the index but unfortunately when I run `setupdb_app.py ... --dropdb --fakedata --fakedata_days=2` it does **not** create the index :(