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/contact.rs

@@ -1179,7 +1179,7 @@ impl Contact {
         Ok(ret)
     }
 
-    /// Adds blocked mailinglists as contacts
+    /// Adds blocked mailinglists and broadcast channels as pseudo-contacts
     /// to allow unblocking them as if they are contacts
     /// (this way, only one unblock-ffi is needed and only one set of ui-functions,
     /// from the users perspective,
@@ -1188,15 +1188,20 @@ impl Contact {
         context
             .sql
             .transaction(move |transaction| {
-                let mut stmt = transaction
-                    .prepare("SELECT name, grpid FROM chats WHERE type=? AND blocked=?")?;
-                let rows = stmt.query_map((Chattype::Mailinglist, Blocked::Yes), |row| {
-                    let name: String = row.get(0)?;
-                    let grpid: String = row.get(1)?;
-                    Ok((name, grpid))
-                })?;
+                let mut stmt = transaction.prepare(
+                    "SELECT name, grpid, type FROM chats WHERE (type=? OR type=?) AND blocked=?",
+                )?;
+                let rows = stmt.query_map(
+                    (Chattype::Mailinglist, Chattype::InBroadcast, Blocked::Yes),
+                    |row| {
+                        let name: String = row.get(0)?;
+                        let grpid: String = row.get(1)?;
+                        let typ: Chattype = row.get(2)?;
+                        Ok((name, grpid, typ))
+                    },
+                )?;
                 let blocked_mailinglists = rows.collect::<std::result::Result<Vec<_>, _>>()?;
-                for (name, grpid) in blocked_mailinglists {
+                for (name, grpid, typ) in blocked_mailinglists {
                     let count = transaction.query_row(
                         "SELECT COUNT(id) FROM contacts WHERE addr=?",
                         [&grpid],
@@ -1209,10 +1214,17 @@ impl Contact {
                         transaction.execute("INSERT INTO contacts (addr) VALUES (?)", [&grpid])?;
                     }
 
+                    let fingerprint = if typ == Chattype::InBroadcast {
+                        // Set some fingerprint so that is_pgp_contact() returns true,
+                        // and the contact isn't marked with a letter icon.
+                        "Blocked_broadcast"
+                    } else {
+                        ""
+                    };
                     // Always do an update in case the blocking is reset or name is changed.
                     transaction.execute(
-                        "UPDATE contacts SET name=?, origin=?, blocked=1 WHERE addr=?",
-                        (&name, Origin::MailinglistAddress, &grpid),
+                        "UPDATE contacts SET name=?, origin=?, blocked=1, fingerprint=? WHERE addr=?",
+                        (&name, Origin::MailinglistAddress, fingerprint, &grpid),
                     )?;
                 }
                 Ok(())