R1:02f82c2af5c4

Modularize fulltext indexing of Projects, Subscriptions and Custom Fields

Summary: Ref T9979. This is going to become `FulltextEngine`, but pave the way for that by pulling extensions out of it.

Test Plan:
{F1036624}

- Used `bin/search index Txxx`, saw projects, subscribers and custom fields rebuild in the index.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9979

Differential Revision: https://secure.phabricator.com/D14835