* 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 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.

Messages an introducee receives in a `FINISHED` state are now being
ignored and deleted.

Closes #295

* normal session where both introducees accept
* normal session where the first introducee declines
* normal session where the second introducee declines
* one session where a contact is introduced to herself
* one session where two identities of the same contact
  are introduced to each other

This introduces a new IntroductionAbortedEvent to signal when the
protocol was aborted. It is not yet used in the UI.

It closes #276