ENH: Create pull request when submitting extension

Finish implementing the base logic to crate the github pull request via
SlicerWizard when submitting an extension. This seems to be working for
additions. For updates, we still also want to generate a compare URL for
the extension repository to add to the pull request message.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22968 3bd1e089-480b-0410-8dfb-8563597acbee