Option to completely disable message decoration on levels list, domain and site.

I need to be able to completely disable all message decoration. It would be great to have toggle switches for this on the levels mailing list, domain and Mailman site.

Also, I suggest making "no decorations" the default setting, given that the current defaults cause breakage of existing DKIM signatures. I'd like Postorius to display warnings to this effect when decorations are enabled.

See also: #324 (closed) and #331 (closed)