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
