Code cleanup and type annotations.
Refactor away Response class.

Go for the more straight forward method of calling the new
eduid_idp.mischttp.create_html_response() instead.
Ignore .idea
Implement verify_request_signatures (default: False).

Apparently, signatures on SAML requests are seldomly validated since
it opens the IdP to a trivial DoS where an attacker sends a stream of
SAML requests with bogus signatures. Those cost the attacker nothing
to create, but costs the IdP a lot to try and validate.
Fix type checking bugs.
Provide 'extra' data to error logging for Sentry.
Handle UTF-8 in translated HTML pages.
Try much harder to keep passwords away from crash logs.
Handle absent Accept-Language header.
Implement server test URI /status.

Tests the userdb and authentication backend. Returns the displayName
of the test user.
unpack_either: Mend code to determine HTTP method.
Minor cleanup/documentation.