Centralized key management for new issuers

[sembrat]

Rather than having per-requisite steps for each instructor and/or course to provide a key pair, allow cred-admin to scaffold the creation and centrally store the keypair.

[stuartf]

The plan here is to have cred-admin generate and host a did:web if a did is not provided when creating a group. That way we retain the flexibility of pointing to an external did but gain the ease of just having cred-admin provide one in most cases.