Add a garbage collector for common ngrams

Summary:
Ref T13000. After an ngram is marked as "common", we can delete it from the storage table.

Currently, the only way to get ngrams marked as "common" is to manually run bin/search ngrams, so this has no impact on normal installs.

Test Plan: Ran bin/garbage collect, saw it start chewing through my local Maniphest ngrams table and removing common ngrams.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13000

Differential Revision: https://secure.phabricator.com/D18687