feat: Group and broadcast channel descriptions (#7829)

fix https://github.com/chatmail/core/issues/7766

Implementation notes:

- Descriptions are only sent with member additions, when the description
is changed, and when promoting a previously-unpromoted group, in order
not to waste bandwith.
- Descriptions are not loaded everytime a chat object is loaded, because
they are only needed for the profile. Instead, they are in their own
table, and can be loaded with their own JsonRPC call.

---------

Co-authored-by: iequidoo <117991069+iequidoo@users.noreply.github.com>

src/mimeparser.rs

@@ -186,10 +186,10 @@ pub enum SystemMessage {
     #[default]
     Unknown = 0,
 
-    /// Group name changed.
+    /// Group or broadcast channel name changed.
     GroupNameChanged = 2,
 
-    /// Group avatar changed.
+    /// Group or broadcast channel avatar changed.
     GroupImageChanged = 3,
 
     /// Member was added to the group.
@@ -254,6 +254,9 @@ pub enum SystemMessage {
 
     /// Message indicating that a call was ended.
     CallEnded = 67,
+
+    /// Group or broadcast channel description changed.
+    GroupDescriptionChanged = 70,
 }
 
 const MIME_AC_SETUP_FILE: &str = "application/autocrypt-setup";
@@ -773,6 +776,11 @@ impl MimeMessage {
             self.is_system_message = SystemMessage::MemberAddedToGroup;
         } else if self.get_header(HeaderDef::ChatGroupNameChanged).is_some() {
             self.is_system_message = SystemMessage::GroupNameChanged;
+        } else if self
+            .get_header(HeaderDef::ChatGroupDescriptionChanged)
+            .is_some()
+        {
+            self.is_system_message = SystemMessage::GroupDescriptionChanged;
         }
     }