Move unbox vectors into envelope-js/-spec

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Coming from this discussion #96 (comment) on the db2 pr #96

copying my last comment into here:

aah hmm i think i remember now. i think the vectors here in tribes were just copy pasted from private-group-spec here https://github.com/ssbc/private-group-spec/tree/master/vectors but without the conversion to URIs that happened there at some point.

private-group-spec doesn't seem to be doing much with those vectors either.

i propose:

• remove the vectors from private-group-spec Remove unbox vectors private-group-spec#35
• remove the vector test from unbox.test.js in ssb-tribes Upgrade to db2 #96
• remove the vectors from ssb-tribes Upgrade to db2 #96
• copy the unbox vectors from ssb-tribes to envelope-spec Add group test vectors envelope-spec#23
• test the vectors using the already existing test in envelope-js in ssb-box2 Add test for group decrypt vectors ssb-box2#35
• Add unbox vectors with sigil links instead private-group-spec#36

i'll make an issue for this since it's gonna be hard to track here