Start making it possible to write to mailing lists (#2736)

See #748, #1964 and 3ba4c6718e/draft/mailing_list_managers.md

Also fix #2735: Assign outgoing messages from other devices to the mailing list

src/headerdef.rs

@@ -33,6 +33,7 @@ pub enum HeaderDef {
     XMozillaDraftInfo,
 
     ListId,
+    ListPost,
     References,
     InReplyTo,
     Precedence,