Enable node revisions for all node types.

Using node revisions would be very helpful for a few reasons:

• Tracking settings changes over time, and who made those changes.
• Keeping track of a site's platform over time. (Currently there is no history of what platforms a site has been on.)
• Other reasons I can't think of right now.

We should enable node revisions for all content types, and edit the code to track VID instead of NID.