refactor: make memberlist update logic easier to follow

[link2xt]

No description provided.

[link2xt]

This does not change anything, just some refactoring I did while debugging #6445

I think it makes it easier to verify that in case there is no Chat-Group-Member-Timestamps header, for non-DC MUA missing members are added from the to field and for DC only explicit changes are allowed and the to field is not used at all.

[Hocuri]

BTW, another readability improvement would be to extract the computation of the group changes messages:

        removed_ids.remove(&removed_id);
    }
    let group_changes_msgs =
        implicit_group_changes_msgs(context, added_ids, removed_ids, chat_id).await?;

    if let Some(avatar_action) = &mime_parser.group_avatar {

and

async fn implicit_group_changes_msgs(
    context: &Context,
    added_ids: HashSet<ContactId>,
    removed_ids: HashSet<ContactId>,
    chat_id: ChatId,
) -> Result<Vec<String>> {
    let mut group_changes_msgs = Vec::new();
    if !added_ids.is_empty() {
        warn!(
            context,
            "Implicit addition of {added_ids:?} to chat {chat_id}."
        );
    }
    if !removed_ids.is_empty() {
        warn!(
            context,
            "Implicit removal of {removed_ids:?} from chat {chat_id}."
        );
    }
    group_changes_msgs.reserve(added_ids.len() + removed_ids.len());
    for contact_id in added_ids {
        let contact = Contact::get_by_id(context, contact_id).await?;
        group_changes_msgs.push(
            stock_str::msg_add_member_local(context, contact.get_addr(), ContactId::UNDEFINED)
                .await,
        );
    }
    for contact_id in removed_ids {
        let contact = Contact::get_by_id(context, contact_id).await?;
        group_changes_msgs.push(
            stock_str::msg_del_member_local(context, contact.get_addr(), ContactId::UNDEFINED)
                .await,
        );
    }

    Ok(group_changes_msgs)
}