Use CouchDocument from new backend (#4475).
Ignore encoding, use message.as_string directly.

The message is already in str type, so we don't care about
encoding. json.dumps will ignore convertion.
Merge remote-tracking branch 'chiiph/bug/multiple_encoding' into develop
Merge remote-tracking branch 'refs/remotes/drebs/bug/4475_use-couch-document' into develop

Conflicts:
	src/leap/mx/mail_receiver.py
Merge branch 'develop' into debian-0.5.0-rc

Preparing for 0.5.0 release
update changelog to 0.5.0
freeze debian version to 0.5.0-rc
bump requirements to unified release