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/stock_str.rs

@@ -237,9 +237,6 @@ pub enum StockMessage {
     #[strum(props(fallback = "Messages"))]
     Messages = 114,
 
-    #[strum(props(fallback = "Broadcast List"))]
-    BroadcastList = 115,
-
     #[strum(props(fallback = "%1$s of %2$s used"))]
     PartOfTotallUsed = 116,
 
@@ -1221,12 +1218,6 @@ pub(crate) async fn part_of_total_used(context: &Context, part: &str, total: &st
         .replace2(total)
 }
 
-/// Stock string: `Broadcast List`.
-/// Used as the default name for broadcast lists; a number may be added.
-pub(crate) async fn broadcast_list(context: &Context) -> String {
-    translated(context, StockMessage::BroadcastList).await
-}
-
 /// Stock string: `⚠️ Your email provider %1$s requires end-to-end encryption which is not setup yet. Tap to learn more.`.
 pub(crate) async fn unencrypted_email(context: &Context, provider: &str) -> String {
     translated(context, StockMessage::InvalidUnencryptedMail)