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/param.rs

@@ -140,7 +140,7 @@ pub enum Param {
     Arg4 = b'H',
 
     /// For Messages
-    AttachGroupImage = b'A',
+    AttachChatAvatarAndDescription = b'A',
 
     /// For Messages
     WebrtcRoom = b'V',
@@ -219,6 +219,9 @@ pub enum Param {
     /// For Chats: timestamp of group name update.
     GroupNameTimestamp = b'g',
 
+    /// For Chats: timestamp of chat description update.
+    GroupDescriptionTimestamp = b'6',
+
     /// For Chats: timestamp of member list update.
     MemberListTimestamp = b'k',