Add 'mw-disambig' CSS class to disambiguation pages

Finally, users and tools for fixing links to disambiguation pages will
be able to distinguish these links without additional API queries.

* Only a single, fast query to get the status of all links on a page.
* Using the GetLinkColours hook, which lets an extension add
  additional CSS classes to all links on a page as it's being parsed.
  The interface it provides is rather silly, but it's sufficient.
* Uncommented the $wgDisambiguatorIndicateLinks configuration setting,
  which allows one to disable this functionality.
* The CSS class probably won't be added on any special pages etc., as
  the only code path to the hook leads through the Parser.
* No styles are provided, only the class, like 'mw-redirect' in core.

Bug: 8339
Change-Id: I72768bf44e0bb34e17e223a74e3eec72f545eba7