If the user has already declined, we don't show that the other
introducee has declined as well. The backend doesn't have that information, so
this is compatible with the principle of showing what we know.
If the user has already accepted or hasn't yet responded, we now show the
decline response in the private conversation with the introducer. If
the user hasn't yet responded, we hide the accept/decline buttons
in the introduction request message.
Please note that I do not have three devices at the moment to test this MR in its entirety in practice. I created another test which is hopefully sufficient to ensure that the modifications are correct.