Optimize Autocrypt gossip

Update gossiped_timestamp when someone else sends autocrypt gossip in
the group, so we postpone sending gossip again ourselves.

- Warn about failures to parse Autocrypt-Gossip header
- Move gossip-related methods into ChatId impl
- Fix a "gossi_pp_ed" typo

src/mimefactory.rs

@@ -361,7 +361,7 @@ impl<'a> MimeFactory<'a> {
         match &self.loaded {
             Loaded::Message { chat } => {
                 // beside key- and member-changes, force re-gossip every 48 hours
-                let gossiped_timestamp = chat.get_gossiped_timestamp(context).await?;
+                let gossiped_timestamp = chat.id.get_gossiped_timestamp(context).await?;
                 if time() > gossiped_timestamp + (2 * 24 * 60 * 60) {
                     Ok(true)
                 } else {