Support Bendy Butt & Gabby Grove feed formats

[mplorentz]

Gabby grove is an alternative feed format which we used to support. Support for it was disabled in #425 due to large changes in the way feed and message identifiers are formatted, but we could bring it back.

[rabble]

We should consider supporting bamboo instead / in addition? Gabby Grove has a bunch of nice features but i think only go-ssb supports it.

[boreq]

Just to clarify this is what the protocol documentation states:

Clients, for compatibility reasons, have a main classic feed alongside the root metafeed.

This implies that every client should have a classic feed as their main feed. What does this mean in the context of gabby grove or bamboo? I thought that those are only supposed to exist in the form of metafeeds.

[rabble]

This will be addressed in scuttlego and specs need to defined.