R1:b54adc6161c2

Kick off indexing for File objects on creation

Summary: Ensures that newly-made `File` objects get indexed into the new ngrams index. Fixes T8788.

Test Plan:
- uploaded a file with daemons stopped; confirmed no new rows in ngrams table
- started daemons; confirmed indexing of previously-uploaded files happened
- uploaded a new file with daemons running; confirmed it got added to the index

Not sure how to test the changes to `PhabricatorFileUploadSource->writeChunkedFile()` and…