Code cleanup: Moved constant maps to wikitext.constants.js

* Fixed a lot of "nodeName in {...}" patterns by moving the {...}
  to wikitext.constants.js where they are easily nameable and fixed
  use sites.

* Moved constants out of mediawiki.Util.js into wikitext.constants.js

* Moved non-Parsoid specific utility functions into jsutils.js and
  used it to break circular dependencies.  In future, more functions
  can be migrated into this file.

Change-Id: I574ae735c7c68fcca53f5eed981cace4f7c5e9b1