Only update interacted_at on new comments

I got feedback from many users, that they don't understand how the "my
activity" stream is sorted, because they have posts on the top, but
don't see why. The user doesn't see if a like was added, and it is also
not imported for the user to see the post again then. And we have
notifications if someone likes a users own posts, so no need to move it on
top of the "my activity" stream here too.

closes #7030