implement crosspost

[estebanabaroa]

there are 2 possible ways to do crosspost, either we would have something like comment.link = 'plebbit://p/name.eth/c/' and plebbit-js doesn't have to do anything. I dont think this option is good, because it forces the mods / subs to download the cid to moderate it

the other way would be to have comment.crosspost = Comment, we put the full comment inside the comment, so the sub owner and mods can moderate it as if it was the original comment, plebbit-js would have to validate the comment to make sure the signature is correct and that it doesnt contain illegal fields.

one possible problem with this is that if you have a chain of crosspost, the comment ipfs becomes large and unbounded, but I dont see a problem with it, a large comment or a large comment chain just wont be possible to crosspost.

another problem is that certain UIs display "crossposted to a, b, c". how would we know this? there are 2 ways, either we only use the subs the user is subscribed to, or we publish crossposts as replies. for example whenever you create comment that is a crosspost, we could simultaneously publish a reply to the crossposted comment, with a link to our crosspost, something like that. this is low priority, we should start with just implementing crossposts first and not worry about how to gather all the "crossposted to a, b, c"