Introduce mediawiki.skinning module for external link icons

Introduce mediawiki.skinning module for external link icons

This should facilitate the creation of third-party skins.

Previously third-party skin creators have had to either copy the relevant
lines from Monobook's main.css, which has led to unnecessary code
duplication and bitrot (to a degree), or no support for external link
icons at all.

This change set will hopefully make the main.css file of your average
custom skin about 86 lines shorter, as well as keep the external link code
and icons centralized for proper RTL support and whatnot (which is *not*
a given when it comes to custom, non-core skins!).

HTTPS' custom icon was dropped and HTTPS URLs are now given the standard
external link arrow icon. This is an intentional design decision.

New icons are Isarra's handiwork, kudos!

Bug: 63521
Change-Id: I8ecbc827f2b05aec79401ac4962fcc15387c8ad8