Naively count rows for large tables to speed UI significantly

Monkeypatch Kaminari::ActiveRecordRelationMethods to naively count
configured tables via reltuples, which - while imprecise - returns
almost instantly as opposed to taking multiple minutes.

This feature defaults to only intercept counts on  `infringing_urls`,
you can configure it by modifying the following ENV with a
comma-separated list:

    TABLES_USING_NAIVE_COUNTS="infringing_urls,works"

See also:

* https://wiki.postgresql.org/wiki/Slow_Counting
Merge pull request #213 from berkmancenter/djcp-intercept-count-star-queries

Naively count rows for large tables to speed UI significantly