GitHub OAuth app/token pool updates

In order for shields Shields to support integrating with GitHub's new(ish) Package Registry offering, we'll need the GitHub tokens in our pool to have the read:packages permission (which they currently do not).

Some action items:

• Transferring the current OAuth App. There's been past discussion about transferring the OAuth App Shields.io uses to Shields org and I think it probably makes sense to do that as part of this work
• Determine how to best get the required token permissions for Shields.io. There's relevant discussion over in #4101 (starting at https://github.com/badges/shields/pull/4101#issuecomment-536309798 and continuing below)