Make ImagePage#getThumbPrevText and ImagePage#makeSizeLink protected instead of private

Make ImagePage#getThumbPrevText and ImagePage#makeSizeLink protected instead of private

So that extensions wishing to modify ImagePage#openShowImage() that want
to preserve most of the existing functionality (at least in certain
contexts) do not have to copypaste these two methods over.
ImagePage#openShowImage() itself is ridiculously large and it's absurd
that to change a few things there you need to copy over the whole giant
block of code, but that's a separate matter.

Inspired by wikiHow's WikihowImagePage extension and their associated core
hacks to ImagePage.php, which hopefully won't be needed in the future.

Change-Id: Ic5e16acfc2cf4683154a60a3eaa9df69d30d03cb