Parser: Allow disabling magic link functionality

Parser: Allow disabling magic link functionality

The magic link functionality is "old backwards-compatibility baggage"
that we probably want to get rid of eventually. The first step to doing
so would be making it configurable and allowing it to be turned off on
wikis that don't use it.

This adds each of the 3 magic link types as individual parser options,
which can be controlled by the $wgEnableMagicLinks setting.

Additionally, wfEscapeWikiText() was updated to only escape enabled
magic link types.

Bug: T47942
Change-Id: If63965f31d17da4b864510146e0018da1cae188c