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)

Start by reading the discussion in #4101, especially the linked pull-request comments, then review the current GitHub OAuth App and token-pool setup. Done means deciding whether to transfer the OAuth App to the Shields organization and ensuring the pool can provide tokens with the read:packages permission.

Written by the indexing model from the issue text.