make OAuth Adapter RFC-compliant

RFC 6749 §2.3.1 states: "The client MUST NOT use more than one
authentication method in each request." Currently, writefreely always
sends both thus requiring a compliant OAuth provider to reject the
request. Add an option to send the credentials in the body, if required
and disable Basic Auth in this case.

Link: https://datatracker.ietf.org/doc/html/rfc6749#section-2.3
Fixes: #1241
Signed-off-by: Moritz Poldrack <git@moritz.sh>