Stop mangling $_GET and provide WebRequest::getQueryValuesOnly()

I doubt there was ever a good reason for mangling $_GET to add the
title, this was just b/c for the sake of b/c. It was formerly used in
core but that was so long ago that I doubt there was any usage in
extensions at the time. Now there is one usage of $_GET['title'] in an
unmaintained extension, but it was only added in 2017.

Also I added WebRequest::getQueryValuesOnly() which is an interface to
the unmodified $_GET. The motivation is allowing OAuth to work with the
REST API, since OAuth needs an unmangled view of $_GET for signature
generation. The Action API gets around the problem with a special hack
in interpolateTitle(), disabling it for the Action API only.

A review of callers of getQueryValues() suggests that many would
benefit from using getQueryValuesOnly() instead. But I only changed it for
callers in api.php and thumb.php since the effect of the change there is
certainly beneficial, whereas callers under index.php may possibly be using
the path parameters to construct self-links.

Rest\RequestFromGlobals uses $_GET directly, which means that this
change causes it to not return PathRouter matches as GET parameters
anymore.

Change-Id: Ic469577fae17c0b1ac69466df7bc9f03e61c74e3