Less talk about "server"

Because the WebODF editing is not needing a server for collaboration, but could also do P2P e.g. with WebRTC, I propose to not use the term "server" for the host of a session in general, to prevent server-only thinking.

Not sure about the best lingo though. For a start:
* what about renaming the "server/" folder to "backends/" (or "backend/"?)?
* what about talking about "master session" when the central session object somewhere is meant which holds the one true version of all data of the session? Even in P2P there needs to be one central entity which would be meant with "the" current document.
Hm. Keeping that out of this PR for now. But jsglobal should be mentioned soon, need to think more about that (and the name ;) ).