Revert visibility change to HTMLForm $typeMappings

Change Id5668141835e22e3e1b043109e50d73ccd6b1229 changed the visibility
of static $typeMappings to private.  It turns out this breaks
http://www.blue-spice.org/ who want to add their own HTMLForm types and
have no way to extend or override this except by changing it.  So set it
to public static.

As https://www.mediawiki.org/wiki/Manual:Coding_conventions/PHP says,
"It's generally a good idea to avoid visibility changes unless you're
making changes to the function which would break old uses of it anyway."
That trumps PHP CodeSniffer warnings.

Change-Id: Ife82596e96a4650b977422aae6b32f24a392d5fb
Merge "Revert visibility change to HTMLForm $typeMappings"