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/message.rs

@@ -1005,6 +1005,7 @@ impl Message {
     pub async fn get_info_contact_id(&self, context: &Context) -> Result<Option<ContactId>> {
         match self.param.get_cmd() {
             SystemMessage::GroupNameChanged
+            | SystemMessage::GroupDescriptionChanged
             | SystemMessage::GroupImageChanged
             | SystemMessage::EphemeralTimerChanged => {
                 if self.from_id != ContactId::INFO {