fix: Return from dc_get_chatlist(DC_GCL_FOR_FORWARDING) only chats where we can send (#4616)

I.e. exclude from the list the following chats as well:
- Read-only mailing lists.
- Chats we're not a member of.

But as for ProtectionBroken chats, we return them, as that may happen to a verified chat at any
time. It may be confusing if a chat that is normally in the list disappears suddenly. The UI need to
deal with that case anyway.

src/receive_imf/tests.rs

@@ -8,7 +8,7 @@ use crate::chat::{
 };
 use crate::chat::{get_chat_msgs, ChatItem, ChatVisibility};
 use crate::chatlist::Chatlist;
-use crate::constants::DC_GCL_NO_SPECIALS;
+use crate::constants::{DC_GCL_FOR_FORWARDING, DC_GCL_NO_SPECIALS};
 use crate::imap::prefetch_should_download;
 use crate::message::Message;
 use crate::test_utils::{get_chat_msg, TestContext, TestContextManager};
@@ -793,6 +793,8 @@ async fn test_github_mailing_list() -> Result<()> {
 
     let chats = Chatlist::try_load(&t.ctx, 0, None, None).await?;
     assert_eq!(chats.len(), 1);
+    let chats = Chatlist::try_load(&t.ctx, DC_GCL_FOR_FORWARDING, None, None).await?;
+    assert_eq!(chats.len(), 0);
     let contacts = Contact::get_all(&t.ctx, 0, None).await?;
     assert_eq!(contacts.len(), 0); // mailing list recipients and senders do not count as "known contacts"