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
• 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