mediawiki.action.edit.preview: Assorted fixes

* Fix some typos
* Load mediawiki.action.history.diff as a RL dependency, not via
  mw.loader.load
* Get rid of inappropriate uses of .prop()

Change-Id: I4ff0fd1f946644ac651fe6c7b645236124d6d770
Merge "mediawiki.action.edit.preview: Assorted fixes"