ENH: Use 'git credentials' in GithubHelper.py

Change GithubHelper.py to use 'git credentials' to get the login
information for github. This way we are not only handling this
information in the same manner as git, but we leverage its credentials
storage system as well. (The downside is that the user name is no longer
automatically stored permanently if the user is not using a password
manager.)

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22975 3bd1e089-480b-0410-8dfb-8563597acbee