Re-addition add-member messages should be published on the additions feed instead

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Nice optimization to have them on the old epochs, in theory, but it increases complexity that we can't handle right now. Doing it this way will fix 2 issues:

1. Can't re-add people to a new epoch if they haven't posted to the old epoch, since triangulation fails. Found here People not removed in an exclude should be able to post on the new epoch #78 (comment)
2. There's a bug in box2 where triangulation fails if you create your feed after you find the other person's. If we just rely on the additions feed (which we create as soon as possible) this should fix itself People not removed in an exclude should be able to post on the new epoch #78 (comment)

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2. hopefully doesn't even bite us in prod, since people should only replicate additions feeds once we replicate metafeeds, and people will probably only do that once they themselves have groups/metafeeds and therefore called start() and have their own additions feed

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#85 seems to have fixed all the issues in #78

@mixmix you might be unblocked already. if not then you might have to wait for #78