Fix bugs and improve output

* Properly escape the article name in the url.
* Don't use a user login to get VisualEditor to load,
  instead just load the module from ResourceLoader directly
  with mw.loader
  We doesn't require user preferences, session or even an account.
* Use a more safer file name maker.
* Don't use mw.getConfig, that's an ancient compatibility
  layer from one of the extensions deployed on en.wikiedia.org
  it won't work on other wikis and is not from MediaWiki itself.
  Use mw.config.get instead.
* Cache result of ".ve-init-mw-viewPageTarget-saveDialog" query
  instead of quering the DOM 5 times.
* Add empty "diff/" directory to repository to make it work
  by default.