Fix #658: Support blind key rotation in HTTP Signatures

Closes #658 (closed)

Cf https://blog.dereferenced.org/the-case-for-blind-key-rotation

When we fail on an invalid signature while authenticating an HTTP request, we'll now:

1. Catch the error
2. Refetch the actor object (to potentially load a new public key)
3. Retry the signature verification with the new public key (and fail for real this time in case of error)

Todo:

• Regenerate local actor key when a Delete activity occur