Add some class constants to PhutilEditorConfig
Summary: Ref T5105. Add some class constants to the PhutilEditorConfig class to avoid hardcoded strings everywhere.
Test Plan: See D11458.
Reviewers: #blessed_reviewers, epriestley
Reviewed By: #blessed_reviewers, epriestley
Subscribers: Korvin, epriestley
Maniphest Tasks: T5105
Differential Revision: https://secure.phabricator.com/D12913