What activity represents sharing objects? #186
Description
Since #185 has already pulled out the concept of maybe adding a shares attribute; this leaves open the question, how do we represent "sharing" or "boosting" (or if you absolutely must, "retweeting") a message? It's an extremely common social activity.
I believe we had a Share ActivityStreams and dropped it in favor of Announce. IMO that's a mistake since Announce can be easily overloaded (the example shown doesn't look like a Share).
We can settle on Announce as that verb definitively in AP, but we should document it. Anyway, if #185 is happening either in a way that issues a new CR or adds an extension, this issue should follow that one's lead.