Add a footer to outgoing messages that point to where sender keys can be feched. Closes #4526.
Add a header that reflects the validity of incoming signatures. Closes #4354.
Add the preference parameter to openpgp header. Closes #3878.
Fix smtp tests to accept deferred and new param encoding.
Merge remote-tracking branch 'drebs/feature/4526_4354_header-and-footer' into develop