GitHub lets an administrator regenerate an app's private key and revoke

the old one. When that happens the key Review Board has stored can no

longer sign app JWTs, which breaks every installation of the app. There

was no way to recover short of recreating the whole app, since the

app-record account is hidden and has no credential-editing UI.
This adds a "Rotate private key" action to the GitHub App connection. It

accepts a freshly-generated PEM key, validates that it is a usable RSA

private key, and stores it on the app-record account, restoring the

connection in place.

Ran unit tests.
Generated a new private key and uploaded it successfully.