Introduce Special:RedirectExternal

Introduce Special:RedirectExternal

Special:RedirectExternal is an unlisted special page that accepts a URL as
the first argument, and redirects the user to that page.
Example: Special:RedirectExternal/https://mediawiki.org

At the moment, this is intended to be used by the GrowthExperiments project in
order to track outbound visits to certain external links. But it could be
extended in the future to provide parameters for showing a message to the user
before redirecting, or explicitly requiring a user to click on the link, which
could help improve security when users follow on-wiki links to off-wiki sites.

Bug: T207115
Change-Id: I822af14a84569aab22249e2f16a662a60e60f76a