Improve authentication with the GitHub App to dev-desktop

[pietroalbini]

This PR improves git-credential-dev-desktop to the point that it successfully works (even though the error messages are still quite poor when GitHub permissions are missing):

• The scripts have been moved to /usr/local/bin and the confiuration to /etc/github-app-credentials.
• The two scripts have been merged together, removing the CLI to get the token of an arbitrary user.
• Permissions have been fixed.
• A type error in the script has been fixed.
• Python isolation mode has been enabled to mitigate privilege escalation.
• Better error messages have been added when the app is not installed.
• The resiliency of the parsing has been improved.