When selecting Ferret ngrams, select term ngrams (not raw ngrams) for term search

Summary:
Ref T12819. For queries like v0.2, we would incorrectly search for ngrams including 0.2, but this is only a substring ngram: the term corpus splits this into v0 and 2, so 0.2 is not in the ngrams table.

When executing term queries, search for term ngrams instead. This makes "v0.2" work properly again.

Test Plan: Searched for "v0.2", found a task with "v0.2" in the title.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12819

Differential Revision: https://secure.phabricator.com/D18581