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/headerdef.rs

@@ -60,6 +60,9 @@ pub enum HeaderDef {
     ChatGroupName,
     ChatGroupNameChanged,
     ChatGroupNameTimestamp,
+    ChatGroupDescription,
+    ChatGroupDescriptionChanged,
+    ChatGroupDescriptionTimestamp,
     ChatVerified,
     ChatGroupAvatar,
     ChatUserAvatar,