fix: Add tolerance to MemberListTimestamp (#5366)

Let's add a 1-minute tolerance to `Params::MemberListTimestamp`.

This adds to the group membership consistency algo the following properties:
- If remote group membership changes were made by two members in parallel, both of them are applied,
  no matter in which order the messages are received.
- If we remove a member locally, only explicit remote member additions/removals made in parallel are
  allowed, but not the synchronisation of the member list from "To". Before, if somebody managed to
  reply earlier than receiving our removal of a member, we added it back which doesn't look good.

src/mimefactory.rs

@@ -93,7 +93,6 @@ pub struct RenderedEmail {
     // pub envelope: Envelope,
     pub is_encrypted: bool,
     pub is_gossiped: bool,
-    pub is_group: bool,
     pub last_added_location_id: Option<u32>,
 
     /// A comma-separated string of sync-IDs that are used by the rendered email
@@ -614,8 +613,6 @@ impl<'a> MimeFactory<'a> {
             ));
         }
 
-        let mut is_group = false;
-
         if let Loaded::Message { chat } = &self.loaded {
             if chat.typ == Chattype::Broadcast {
                 let encoded_chat_name = encode_words(&chat.name);
@@ -623,8 +620,6 @@ impl<'a> MimeFactory<'a> {
                     "List-ID".into(),
                     format!("{encoded_chat_name} <{}>", chat.grpid),
                 ));
-            } else if chat.typ == Chattype::Group {
-                is_group = true;
             }
         }
 
@@ -896,7 +891,6 @@ impl<'a> MimeFactory<'a> {
             // envelope: Envelope::new,
             is_encrypted,
             is_gossiped,
-            is_group,
             last_added_location_id,
             sync_ids_to_delete: self.sync_ids_to_delete,
             rfc724_mid,