Summary: Follows the outline in D15656 for implementing ngram search for names of File objects. Also created FileFullTextEngine, because without implementing `PhabricatorFulltextInterface`, `./bin/search` complains that `File` is not an indexable type. Test Plan: - ran `./bin/storage upgrade` to apply the schema change - confirmed the presence of a new `file_filename_ngrams` table - added a couple file objects - ran `bin/search index --type file --force` - confirmed the presence of rows in `file_filename_ngrams` - did a few keyword searches and saw expected results Reviewers: epriestley Reviewed By: epriestley Subscribers: Korvin Maniphest Tasks: T8788 Differential Revision: https://secure.phabricator.com/D17702