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