Links created by Linker::makeExternalLink didn't include rel=nofollow

The links created by Linker::makeExternalLink didn't obey $wgNoFollowLinks
(and $wgNoFollowNsExceptions, $wgNoFollowDomainExceptions). To
fix this I made the function call Parser::getExternalLinkRel which
I implemented earlier.

Fixes bug 41983.

Change-Id: I2ff35f89502db2e1f8266f3a943e38c0ea67aced
Merge "Links created by Linker::makeExternalLink didn't include rel=nofollow"