R1:976fbee877c9

Implement ngram search for File objects

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`…