SECURITY: Always normalize link url before adding to ParserOutput

SECURITY: Always normalize link url before adding to ParserOutput

Move link normalization directly into addExternalLink() method,
since you always need to do it - having it separate is just
inviting people to forget to normalize a link.

Additionally, links weren't properly registered for <gallery>.
This was somewhat unnoticed, as the call to recursiveTagParse()
would register free links, but it wouldn't work for example with
protocol relative links.

Issue originally reported by MZMcBride.

Bug: T48143
Change-Id: I557fb3b433ef9d618097b6ba4eacc6bada250ca2