Triangles should be constructed when new feeds are discovered

[Powersource]

Context (and even code suggestion for the fix) ssbc/ssb-tribes2#78 (comment)

Note that this is an actual bug in box2. There's a separate design flaw in DMing, that you can only DM someone that has a mirror feed to you. This bug is about box2 failing to DM even when there is a mirror feed, and this should be fixed.

However since we moved re-additions to the additions feed as well, this is not as urgent anymore and we might (not entirely sure) be able to put private groups in production without having to fix this.