fix: Update the channel title after joining if the QR code included a wrong title (#8260)

Fix https://github.com/chatmail/core/issues/8250. Not sure why anyone
would tamper with the link, but maybe the name gets ellipsized because
it is very long, or maybe we decide to ellipsize more aggressively in
the future.

The fix is easy enough, just set the GroupNameTimestamp when creating
the channel, so that `HeaderDef::ChatGroupNameTimestamp` gets set when
sending a message and the logic in
`apply_chat_name_avatar_and_description_changes()` recognizes the
incoming name's timestamp as newer than `chat_group_name_timestamp`
(which is 0 right after joining a chat).

The test is somewhat hacky and depends on the title being the last
parameter in the invite code URL, but this is fine; if we ever change
that, then the test will fail and we need to modify it.

This also removes some wrong doc comments.

Since groups are unpromoted until sending a first message, the same fix isn't needed for groups.

deltachat-jsonrpc/src/api.rs

@@ -1106,9 +1106,6 @@ impl CommandApi {
     /// because the word "channel" already appears a lot in the code,
     /// which would make it hard to grep for it.
     ///
-    /// After creation, the chat contains no recipients and is in _unpromoted_ state;
-    /// see [`CommandApi::create_group_chat`] for more information on the unpromoted state.
-    ///
     /// Returns the created chat's id.
     async fn create_broadcast(&self, account_id: u32, chat_name: String) -> Result<u32> {
         let ctx = self.get_context(account_id).await?;