(groups): Don't forward Alice's metafeed tree to Bob if Alice blocks Bob

[staltz]

Say I follow both Alice and Bob, but Alice blocks Bob.

It's currently a feature in ssb-replication-scheduler that we won't forward Alice's (main) feed to Bob, and it's implemented here:

ssb-replication-scheduler/index.js

Line 49 in 6a01b2f

ssb.ebt.block(source, dest, value === -1)

But that's only for main feeds. Bob is still capable of replicating Alice's metafeed tree.

We should support preventing Bob from getting Alice's metafeed tree (any of Alice's feeds).

But there is an exception to this: Bob can always get Alice's group leaf feed (and the whole branch above that) if both Alice and Bob are in the same group.