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.

src/blob/blob_tests.rs

@@ -4,7 +4,7 @@ use super::*;
 use crate::message::{Message, Viewtype};
 use crate::param::Param;
 use crate::sql;
-use crate::test_utils::{self, TestContext};
+use crate::test_utils::{self, AVATAR_64x64_BYTES, AVATAR_64x64_DEDUPLICATED, TestContext};
 use crate::tools::SystemTime;
 
 fn check_image_size(path: impl AsRef<Path>, width: u32, height: u32) -> image::DynamicImage {
@@ -241,9 +241,8 @@ async fn test_selfavatar_in_blobdir() {
 async fn test_selfavatar_copy_without_recode() {
     let t = TestContext::new().await;
     let avatar_src = t.dir.path().join("avatar.png");
-    let avatar_bytes = include_bytes!("../../test-data/image/avatar64x64.png");
-    fs::write(&avatar_src, avatar_bytes).await.unwrap();
-    let avatar_blob = t.get_blobdir().join("e9b6c7a78aa2e4f415644f55a553e73.png");
+    fs::write(&avatar_src, AVATAR_64x64_BYTES).await.unwrap();
+    let avatar_blob = t.get_blobdir().join(AVATAR_64x64_DEDUPLICATED);
     assert!(!avatar_blob.exists());
     t.set_config(Config::Selfavatar, Some(avatar_src.to_str().unwrap()))
         .await
@@ -251,7 +250,7 @@ async fn test_selfavatar_copy_without_recode() {
     assert!(avatar_blob.exists());
     assert_eq!(
         fs::metadata(&avatar_blob).await.unwrap().len(),
-        avatar_bytes.len() as u64
+        AVATAR_64x64_BYTES.len() as u64
     );
     let avatar_cfg = t.get_config(Config::Selfavatar).await.unwrap();
     assert_eq!(avatar_cfg, avatar_blob.to_str().map(|s| s.to_string()));