Automatically decline incoming introduction requests when they self-destruct

When an incoming introduction request self-destructs without being answered, automatically decline the request. This may require a protocol update (coordinated with #1829 (closed)) to flag the decline as an automatic response that shouldn't be shown in the UI.

Subtask of #804 (closed)

Test instructions:

• Use three devices, users A, B and C
• Enable self-destructing messages in the conversations A-B and A-C
• Let A introduce contacts B and C
• Expect invitation messages to arrive at B and C about the invitation
• Expect the invitation messages to have a auto-delete timers
• Let those timers expire. Expect that to trigger an automatic decline of the invitation, i.e. on all three devices it is visible that the introduction failed (due to the expired response)
• Expect all messages from that interaction to destroy after each message's timer expires
• Let A introduce B and C again. Expect this not to fail due to an introduction that is already going on (because none should be going on any longer)
• Let B and C accept the introduction
• Expect the introduction to work
• Confirm that B and C have each other in the contact list
• Expect all messages involved in the transaction to have auto-delete timers
• Let those timers expire and expect all those messages to disappear