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/receive_imf.rs

@@ -14,7 +14,9 @@ use mailparse::SingleInfo;
 use num_traits::FromPrimitive;
 use regex::Regex;
 
-use crate::chat::{self, Chat, ChatId, ChatIdBlocked, ProtectionStatus};
+use crate::chat::{
+    self, Chat, ChatId, ChatIdBlocked, ProtectionStatus, remove_from_chat_contacts_table,
+};
 use crate::config::Config;
 use crate::constants::{Blocked, Chattype, DC_CHAT_ID_TRASH, EDITED_PREFIX, ShowEmails};
 use crate::contact::{Contact, ContactId, Origin, mark_contact_id_as_verified};
@@ -1687,7 +1689,9 @@ async fn add_parts(
         _ if chat.id.is_special() => GroupChangesInfo::default(),
         Chattype::Single => GroupChangesInfo::default(),
         Chattype::Mailinglist => GroupChangesInfo::default(),
-        Chattype::OutBroadcast => GroupChangesInfo::default(),
+        Chattype::OutBroadcast => {
+            apply_out_broadcast_changes(context, mime_parser, &mut chat, from_id).await?
+        }
         Chattype::Group => {
             apply_group_changes(
                 context,
@@ -1701,7 +1705,7 @@ async fn add_parts(
             .await?
         }
         Chattype::InBroadcast => {
-            apply_broadcast_changes(context, mime_parser, &mut chat, from_id).await?
+            apply_in_broadcast_changes(context, mime_parser, &mut chat, from_id).await?
         }
     };
 
@@ -3438,7 +3442,30 @@ async fn apply_mailinglist_changes(
     Ok(())
 }
 
-async fn apply_broadcast_changes(
+async fn apply_out_broadcast_changes(
+    context: &Context,
+    mime_parser: &MimeMessage,
+    chat: &mut Chat,
+    from_id: ContactId,
+) -> Result<GroupChangesInfo> {
+    ensure!(chat.typ == Chattype::OutBroadcast);
+
+    if let Some(_removed_addr) = mime_parser.get_header(HeaderDef::ChatGroupMemberRemoved) {
+        // The sender of the message left the broadcast channel
+        remove_from_chat_contacts_table(context, chat.id, from_id).await?;
+
+        return Ok(GroupChangesInfo {
+            better_msg: Some("".to_string()),
+            added_removed_id: None,
+            silent: true,
+            extra_msgs: vec![],
+        });
+    }
+
+    Ok(GroupChangesInfo::default())
+}
+
+async fn apply_in_broadcast_changes(
     context: &Context,
     mime_parser: &MimeMessage,
     chat: &mut Chat,
@@ -3459,6 +3486,15 @@ async fn apply_broadcast_changes(
     )
     .await?;
 
+    if let Some(_removed_addr) = mime_parser.get_header(HeaderDef::ChatGroupMemberRemoved) {
+        // The only member added/removed message that is ever sent is "I left.",
+        // so, this is the only case we need to handle here
+        if from_id == ContactId::SELF {
+            better_msg
+                .get_or_insert(stock_str::msg_group_left_local(context, ContactId::SELF).await);
+        }
+    }
+
     if send_event_chat_modified {
         context.emit_event(EventType::ChatModified(chat.id));
         chatlist_events::emit_chatlist_item_changed(context, chat.id);