BUG: Find pull request more strictly

Change how the Extension Wizard looks up pull requests to do the look-up
not against the user that submitted the request, but against the
downstream repository (which should imply the user). I'm not sure if
github even allows a user to have multiple forks of the same upstream
repository, but this will guard against that in case it would ever
happen.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22990 3bd1e089-480b-0410-8dfb-8563597acbee