[development] Requiring node revisions

[Robert Douglass]

So far, David has convinced me that we should make revisions 
automatically and all the time, and that we should work towards 
simplifying the UPDATE/INSERT workflow for nodes. Doing this, and 
simultaneously working on the interface and workflow for revision 
management, will allow us to become a next generation publishing tool.

We gain:
- a very straightforward of saving drafts
- the equivalent of Angie's revision moderation - in core
- easier programming API
- less cluttered UI
- improved preview workflow

I spend a lot of time worrying about performance, yet this change 
doesn't concern me. People who are worried about performance would be 
better of finding a better schema for the block tables so that the block 
loading queries aren't an order of magnitude slower than all the other 
queries that run on any page.