Convert all "DocumentIndexers" into "FulltextEngines"
Summary: Ref T9979. This simplifies/standardizes the code a bit, but mostly gives us more consistent class names and structure.
Test Plan:
- Used bin/search index --type ... to index documents of every indexable type.
- Searched for documents by unique text, found them.
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T9979
Differential Revision: https://secure.phabricator.com/D14842