Cleanup items identified in GuidedTour audit.

Mostly this entails removing code that is no longer necessary,
or that is deprecated.

* Removing custom site styles (GuidedTour-custom.css); this can be
  done with Common.css and the feature does not appear to be commonly
  used (or used on the WMF cluster)

* Removing obsolete comments

* Added CSS prefixing

* Renamed ext.guidedTour.js for clarity

* Removed workaround since CSSJanus file name flipping now works on
  background-image-svg

* Use core mixins where possible

* Remove support for browsers that no longer receive JS (IE 6 and IE 7).
  This involves changing the CSS since all our CSS is only useful with
  JS, and is served with ResourceLoader.

* Remove cleanTourName.  .. should be safe in the query string (although
  it is not supported in tour names by the server for separate names),
  and URL encoding (which should be done immediately before navigating
  to the URL) is not an issue since we are not constructing tour= URLs.

* Use mediawiki.cookie instead of using jquery.cookie directly, so
  prefix and path support are handled automatically.  Also, remove
  convertToNewCookie format since all cookies are most likely
  converted by now.

* Minor refactoring of tour-launching

* Remove recordStats, which was already a no-op.

* Remove a little dead code from the mediawiki.libs.guiders module.

* Remove the i18n shim.

The audit was at https://gerrit.wikimedia.org/r/#/c/164585/

Bug: 70700
Change-Id: I00be8741c1b7e87cbf51ae1d4d3dbeffac8002de
Merge "Cleanup items identified in GuidedTour audit."