Merge master into release 4.0

• I've changed quoting in many cases (' ' vs " ") to make rubocop pass.

• I've added x-stop to all the specs which lacked this keyword, to make them pass.

• I've added 2ff6416c to spec/migrations/remove_keywords_admin_only_spec.rb.

• I've adjusted 711916e8, 64b9b6cf and 6187df38 to the new keyword_handlers.

• Two features were added to master in the meantime: deliver_selfsent and include_autocrypt_header: I've made changes to the DB schema and migration files to account for ActiveRecord 5.2, to some spec files and to open-api-spec.yml.

• I've adjusted 7a3f3c9b to the new keyword_handlers. However, the spec doesn't pass, Schleuder reports Your mail was found empty... if handling the mail which this commit introduced. I couldn't figure out why. I thought about skipping this test for now, to allow to merge this MR as soon as possible, to unblock other work, but probably one of you has an idea regarding this.

• Probably more stuff to mention, which I forgot. It took a bit of time, but see for yourself. I appreciate (critical) reviews. :)