Merge branch '295-show-declined-introductions' into 'master'

Show relevant introduction decline responses in the conversation

* 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.

Closes #295 

See merge request !149