Do not track incoming positive introduction responses

Positive introduction responses are not shown in the UI (for introducees) and are therefore not marked as read. If they would be tracked, the unread message count would be higher than it actually is and would never decrease.

This is a minimal fix that could be better, but I didn't bother to refactor anything, because we need to rewrite the introduction client eventually anyway once more.

Closes #759 (closed)