Support external links in MWInternalLinkAnnotation

As URLs from the clipboard are always absolute, we need to detect if these
are from the same wiki as the current document, and if so convert back to
relative for Parsoid.

Bug: 58136
Change-Id: Id251afe65193fc6356628f1deb5ed757f8a6d347
Merge "Support external links in MWInternalLinkAnnotation"