#1825 REST comments

Use REST when possible to sync, moderate, edit, or reply to comments.

I've been testing this with wp.com, jetpack and non-jetpack self-hosted, and everything seems to work fine, but feel free to look for corner cases and try to break it.

Fixes #1825 
I see these Service classes as a kind of Controller. Just as `RemoteComment` shouldn't know about Core Data, I don't think `Comment` should know about how the networking is done