For backup persitsence, mark the "common ngrams" table as a data table, not an index table

Summary:
Ref T13000. Garbage collecting common ngrams is slow because MySQL isn't all that great at deleting rows quickly. See PHI96, where it looks like it's going to take a week to GC ngrams for a ~million objects at a relatively conservative 0.15 threshold.

In the event of a restore, we can reduce the impact by persisting this table so the ngrams just don't get built when the reindex happens.

Test Plan: Viewed schema in Config, saw common ngrams tables marked as "Data" instead of "Index".

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13000

Differential Revision: https://secure.phabricator.com/D18696