Rename "SearchEngine" to "FulltextStorageEngine"

Summary:
Ref T9979. I picked this name long before the advent of modern "Engine" architecture and it ended up being pretty confusing.

Rename "SearchEngine" (currently: mysql or elasticsearch, used to store and query fulltext indexes) to "FulltextStorageEngine" to make it more clear what it does and disambituate it from ApplicationSearch, which also has a bunch of stuff called "SearchEngine", "SearchEngineExtension", etc.

Test Plan: Grepped for phabricatorsearchengine.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9979

Differential Revision: https://secure.phabricator.com/D14843