Adapt to the new channels

[Hocuri]

When updating core to v2.24.0, which as chatmail/core#7268 merged, some adaptions are needed:

• There is a new QR type DC_QR_ASK_JOIN_BROADCAST = 204 (CFFI) / AskJoinBroadcast (JsonRPC)
  • It should be handled just as DC_QR_ASK_VERIFYGROUP and DC_QR_ASK_VERIFYCONTACT, just that the dialog shown after scanning uses the translated string qrscan_ask_join_channel
• Cloning an outgoing broadcast channel should be impossible
• In the profile view of broadcast channels, Add member should be hidden, and instead QR Invite Code should be shown
• While creating a broadcast channel:
  • the "Add Members"/"Add Recipients" option should be hidden.
  • The translated string chat_new_channel_hint should be shown.
• UI-testing using the list at [Tracking issue] End-to-End encrypted Broadcast Channels chatmail/core#6884

Android PR for reference: deltachat/deltachat-android#3886

[link2xt]

Core 2.24.0 has a blocker bug, do not use it for releases: chatmail/core#7397

[r10s]

2.26 is there, so this issue should be resolved before next release