feat: Show broadcast channels in their own, proper "Channel" chat (#6901)

Part of #6884 

----

- [x] Add new chat type `InBroadcastChannel` and `OutBroadcastChannel`
for incoming / outgoing channels, where the former is similar to a
`Mailinglist` and the latter is similar to a `Broadcast` (which is
removed)
- Consideration for naming: `InChannel`/`OutChannel` (without
"broadcast") would be shorter, but less greppable because we already
have a lot of occurences of `channel` in the code. Consistently calling
them `BcChannel`/`bc_channel` in the code would be both short and
greppable, but a bit arcane when reading it at first. Opinions are
welcome; if I hear none, I'll keep with `BroadcastChannel`.
- [x] api: Add create_broadcast_channel(), deprecate
create_broadcast_list() (or `create_channel()` / `create_bc_channel()`
if we decide to switch)
  - Adjust code comments to match the new behavior.
- [x] Ask Desktop developers what they use `is_broadcast` field for, and
whether it should be true for both outgoing & incoming channels (or look
it up myself)
- I added `is_out_broadcast_channel`, and deprecated `is_broadcast`, for
now
- [x] When the user changes the broadcast channel name, immediately show
this change on receiving devices
- [x] Allow to change brodacast channel avatar, and immediately apply it
on the receiving device
- [x] Make it possible to block InBroadcastChannel
- [x] Make it possible to set the avatar of an OutgoingChannel, and
apply it on the receiving side
- [x] DECIDE whether we still want to use the broadcast icon as the
default icon or whether we want to use the letter-in-a-circle
- We decided to use the letter-in-a-circle for now, because it's easier
to implement, and I need to stay in the time plan
- [x] chat.rs: Return an error if the user tries to modify a
`InBroadcastChannel`
- [x] Add automated regression tests
- [x] Grep for `broadcast` and see whether there is any other work I
need to do
- [x] Bug: Don't show `~` in front of the sender's same in broadcast
lists

----

Note that I removed the following guard:

```rust
        if !new_chat_contacts.contains(&ContactId::SELF) {
            warn!(
                context,
                "Received group avatar update for group chat {} we are not a member of.", chat.id
            );
        } else if !new_chat_contacts.contains(&from_id) {
            warn!(
                context,
                "Contact {from_id} attempts to modify group chat {} avatar without being a member.",
                chat.id,
            );
        } else [...]
```

i.e. with this change, non-members will be able to modify the avatar.
Things were slightly easier this way, and I think that this is in line
with non-members being able to modify the group name and memberlist
(they need to know the Group-Chat-Id, anyway), but I can also change it
back.

deltachat-rpc-client/tests/test_something.py

@@ -11,7 +11,7 @@ from unittest.mock import MagicMock
 import pytest
 
 from deltachat_rpc_client import Contact, EventType, Message, events
-from deltachat_rpc_client.const import DownloadState, MessageState
+from deltachat_rpc_client.const import ChatType, DownloadState, MessageState
 from deltachat_rpc_client.rpc import JsonRpcError
 
 
@@ -846,3 +846,36 @@ def test_delete_deltachat_folder(acfactory, direct_imap):
     assert msg.text == "hello"
 
     assert "DeltaChat" in ac1_direct_imap.list_folders()
+
+
+def test_broadcast(acfactory):
+    alice, bob = acfactory.get_online_accounts(2)
+
+    alice_chat = alice.create_broadcast("My great channel")
+    snapshot = alice_chat.get_basic_snapshot()
+    assert snapshot.name == "My great channel"
+    assert snapshot.is_unpromoted
+    assert snapshot.is_encrypted
+    assert snapshot.chat_type == ChatType.OUT_BROADCAST
+
+    alice_contact_bob = alice.create_contact(bob, "Bob")
+    alice_chat.add_contact(alice_contact_bob)
+
+    alice_msg = alice_chat.send_message(text="hello").get_snapshot()
+    assert alice_msg.text == "hello"
+    assert alice_msg.show_padlock
+
+    bob_msg = bob.wait_for_incoming_msg().get_snapshot()
+    assert bob_msg.text == "hello"
+    assert bob_msg.show_padlock
+    assert bob_msg.error is None
+
+    bob_chat = bob.get_chat_by_id(bob_msg.chat_id)
+    bob_chat_snapshot = bob_chat.get_basic_snapshot()
+    assert bob_chat_snapshot.name == "My great channel"
+    assert not bob_chat_snapshot.is_unpromoted
+    assert bob_chat_snapshot.is_encrypted
+    assert bob_chat_snapshot.chat_type == ChatType.IN_BROADCAST
+    assert bob_chat_snapshot.is_contact_request
+
+    assert not bob_chat.can_send()