Make "mysql" mean "Ferret engine" in Fulltext search
Summary: Ref T12819. Swaps constants so existing configurations that use a "mysql" engine now use the Ferret engine, not an InnoDB/MyISAM FULLTEXT engine.
Test Plan: Swapped my local config back to "mysql" (the default), saw Ferret engine results in the UI.
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T12819
Differential Revision: https://secure.phabricator.com/D18590