Add new members to all tip epochs, and make sure that re-adding is allowed

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We had a discussion on signal and i wrote this:

https://github.com/ssbc/ssb-group-exclusion-spec#49-adding-new-members

Idk if the spec describes what we have in code that well

The code publishes one add per epoch tip

Also "The "correct membership" for a particular epoch is calculated as the union of all declared members in all epochs, minus the union of all excluded members in any epoch up till that epoch." sounds wrong, that disallows re-adding, which is something the module supports

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where this was implemented ssbc/ssb-tribes2#106