feat: Make it possible to leave broadcast channels (#6984)

Part of #6884.
The channel owner will not be notified in any way that you left, they
will only see that there is one member less.

For the translated stock strings, this is what we agreed on in the
group:
- Add a new "Leave Channel" stock string (will need to be done in UIs)
- Reword the existing "Are you sure you want to leave this group?"
string to "Are you sure you want to leave?" (the options are "Cancel"
and "Leave Group" / "Leave Channel", so it's clear what you are leaving)
(will need to be done in the deltachat-android repo, other UIs will pick
it up automatically)
- Reword the existing "You left the group." string to "You left". (done
here, I will adapt the strings in deltachat-android, too)

I adapted DC Android by pushing
6df2740884
to https://github.com/deltachat/deltachat-android/pull/3783.

---------

Co-authored-by: l <link2xt@testrun.org>

src/chat.rs

@@ -2905,10 +2905,12 @@ async fn prepare_send_msg(
             // If the chat is a contact request, let the user accept it later.
             msg.param.get_cmd() == SystemMessage::SecurejoinMessage
         }
-        // Allow to send "Member removed" messages so we can leave the group.
+        // Allow to send "Member removed" messages so we can leave the group/broadcast.
         // Necessary checks should be made anyway before removing contact
         // from the chat.
-        CantSendReason::NotAMember => msg.param.get_cmd() == SystemMessage::MemberRemovedFromGroup,
+        CantSendReason::NotAMember | CantSendReason::InBroadcast => {
+            msg.param.get_cmd() == SystemMessage::MemberRemovedFromGroup
+        }
         CantSendReason::MissingKey => msg
             .param
             .get_bool(Param::ForcePlaintext)
@@ -4079,8 +4081,6 @@ pub async fn remove_contact_from_chat(
         "Cannot remove special contact"
     );
 
-    let mut msg = Message::new(Viewtype::default());
-
     let chat = Chat::load_from_db(context, chat_id).await?;
     if chat.typ == Chattype::Group || chat.typ == Chattype::OutBroadcast {
         if !chat.is_self_in_chat(context).await? {
@@ -4110,19 +4110,10 @@ pub async fn remove_contact_from_chat(
             // in case of the database becoming inconsistent due to a bug.
             if let Some(contact) = Contact::get_by_id_optional(context, contact_id).await? {
                 if chat.typ == Chattype::Group && chat.is_promoted() {
-                    msg.viewtype = Viewtype::Text;
-                    if contact_id == ContactId::SELF {
-                        msg.text = stock_str::msg_group_left_local(context, ContactId::SELF).await;
-                    } else {
-                        msg.text =
-                            stock_str::msg_del_member_local(context, contact_id, ContactId::SELF)
-                                .await;
-                    }
-                    msg.param.set_cmd(SystemMessage::MemberRemovedFromGroup);
-                    msg.param.set(Param::Arg, contact.get_addr().to_lowercase());
-                    msg.param
-                        .set(Param::ContactAddedRemoved, contact.id.to_u32() as i32);
-                    let res = send_msg(context, chat_id, &mut msg).await;
+                    let addr = contact.get_addr();
+
+                    let res = send_member_removal_msg(context, chat_id, contact_id, addr).await;
+
                     if contact_id == ContactId::SELF {
                         res?;
                         set_group_explicitly_left(context, &chat.grpid).await?;
@@ -4141,6 +4132,11 @@ pub async fn remove_contact_from_chat(
                 chat.sync_contacts(context).await.log_err(context).ok();
             }
         }
+    } else if chat.typ == Chattype::InBroadcast && contact_id == ContactId::SELF {
+        // For incoming broadcast channels, it's not possible to remove members,
+        // but it's possible to leave:
+        let self_addr = context.get_primary_self_addr().await?;
+        send_member_removal_msg(context, chat_id, contact_id, &self_addr).await?;
     } else {
         bail!("Cannot remove members from non-group chats.");
     }
@@ -4148,6 +4144,28 @@ pub async fn remove_contact_from_chat(
     Ok(())
 }
 
+async fn send_member_removal_msg(
+    context: &Context,
+    chat_id: ChatId,
+    contact_id: ContactId,
+    addr: &str,
+) -> Result<MsgId> {
+    let mut msg = Message::new(Viewtype::Text);
+
+    if contact_id == ContactId::SELF {
+        msg.text = stock_str::msg_group_left_local(context, ContactId::SELF).await;
+    } else {
+        msg.text = stock_str::msg_del_member_local(context, contact_id, ContactId::SELF).await;
+    }
+
+    msg.param.set_cmd(SystemMessage::MemberRemovedFromGroup);
+    msg.param.set(Param::Arg, addr.to_lowercase());
+    msg.param
+        .set(Param::ContactAddedRemoved, contact_id.to_u32());
+
+    send_msg(context, chat_id, &mut msg).await
+}
+
 async fn set_group_explicitly_left(context: &Context, grpid: &str) -> Result<()> {
     if !is_group_explicitly_left(context, grpid).await? {
         context