feat: send pre-message on messages with large attachments

[Simon-Laux]

part of #7367

progress / what's to do:

• send pre-message
• The message's state must be set to MessageState::OutDelivered only after both messages are sent. If a read receipt is received, the message can be OutMdnRcvd or OutPending; let's just do whatever is easiest for now. Take care not to revert from OutMdnReceived to OutDelivered if we first receive a read receipt and then deliver the full message.
  • this is already the case:
    • OutDelivered is set when a message is sent out and has no remaining send jobs in the smtp table for this message id - so already works since full message and pre message have same msgId in that table.
    • OutMdnRcvd is a "virtual" state ([Tracking Issue] Pre-Messages #7367 (comment)), so going back to OutDelivered can't happen anymore
• delimit ChatFullMessageId with < and > like the other message ids
• add tests
  • test that pre message is sent for attachment larger than X
    • test that correct headers are present on both messages
    • also test that Autocrypt-gossip and selfavatar should never go into full-messages
  • test that no pre message is sent for attachment smaller than X
    • no "is full message" header should be present
  • test that pre message is not send for large webxdc update or large text
• fix test receive_imf::receive_imf_tests::test_dont_reverify_by_self_on_outgoing_msg