Show channels in a proper "Channel" chat (#3783)

* Rename "broadcast list" to "channel"/"broadcast channel" both in UI and code

* feat: Add new channel types

* Update CHANGELOG.md

* adb's review

* refactor: Rename BroadcastChannel to Broadcast

* Revert accidental change

* Make it possible to leave channels

- In a chat, if the chat is an InBroadcast, and it's not a contact
  request, then the `Leave` menu option is shown with the translated
  stock string `menu_leave_channel` as its label.
- If the user clicks on it, the confirmation dialog has
  `menu_leave_channel` (rather than `menu_leave_group`) as its positive
  option.

Counterpart of https://github.com/chatmail/core/pull/6984.

---------

Co-authored-by: adbenitez <asieldbenitez@gmail.com>
Co-authored-by: adb <adb@merlinux.eu>

src/main/java/com/b44t/messenger/rpc/Rpc.java

@@ -160,6 +160,10 @@ public class Rpc {
         getResult("add_or_update_transport", accountId, param);
     }
 
+    public int createBroadcast(int accountId, String chatName) throws RpcException {
+        return gson.fromJson(getResult("create_broadcast", accountId, chatName), Integer.class);
+    }
+
     private static class Request {
         private final String jsonrpc = "2.0";
         public final String method;