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