Initial version of Disambiguator extension

Enables the designation of disambiguation pages with the __DISAMBIG__
magic word. Also provides the following:
* Special Page listing all disambiguation pages
* Special Page listing all pages that link to disambiguation pages
* APIs for both of the above

Solves bug 6754 and bug 35981 and bug 44040

See https://www.mediawiki.org/wiki/Extension:Disambiguator

Change-Id: Idf9e37a576c7bb52c717f754bf09be5809391242