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
