When mastodon is in authorized fetch mode any request has to be signed
or it fails with 401. This adds the needed signature to the requests
made to discover the actor when receiving something from mastodon (such
as a follow request)

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If no digest value is passed to make_signature and Exception was thrown.
Since digest is added to the signature headers if it is not None anyway, there is no need to assign the digest value before that check.
When signing a request _as the server_ for Mastodon's AUTHORIZED_FETCH there is no need to include a digest.

ERROR HANDLING FIXES

- use raise_for_status() to pass through response code
- handle exceptions where no response object is passed through

INSTANCE ACTOR

- models.User.objects.create_user function cannot take an ID
- allow instance admins to determine username and email for instance actor in settings.py

Makes exception handling more precise, only raising status for 401s.

Also fixes a string pylint was complaining about.

- when using raise_for_status we need to catch an HTTPError, not a ConnectionError
- simplify instance actor - use internal email address since it will never be used anyway, and make default username less likely to already be in use.

I can't even tell what it thinks it did, but Black likes to complain.