Extension:CentralNotice/Notes/Extension registration

Notes for finalizing the conversion of CentralNotice to use extension registration. Current work is here:.

Copied code to verify

 * Autoloaded classes ✅ (PS 33)
 * API modules and list modules ✅ (PS 33)
 * Hooks ( and  ) ✅ (PS 35)
 * Special pages ✅ (PS 33)
 * RL modules ✅ (PS 33)
 * Config variables and defaults ✅ (PS 33)

Functionality to smoke test

 * UserMerge integration
 * TranslationExtension integration
 * User rights
 * Namespace registration
 * RL loading (modules loaded on most pages vs. those on Special and edit pages)
 * Variables sent to JS
 * Presence of SiteNotice div
 * PHP and JS unit tests
 * EventLogging schema
 * API