Title::getLinkURL(): Allow expanding PROTO_RELATIVE too

Title::getLinkURL(): Allow expanding PROTO_RELATIVE too

Allow callers to get protocol-relative URLs by passing the
PROTO_RELATIVE constant. Since most callers don't pass in $proto, this
shouldn't change very much since the default of non-expanded is
effectively the same, but expanding to a protocol-relative URL when it
is not intended should still be safe.

A few callers that explicitly specified the default in core were also
updated.

Change-Id: I214f17aa6e8e30b647f21f0951fc3a15e87850c7