Add table to track node revision_moderation status

It's been kind of universally agreed that combining "in moderation" and "create new revision" statuses a) won't work at all in 5.0 since we don't have "in moderation" anymore and b) is counter-intuitive. Therefore, we should add a new state "revision moderation" to the list of workflow states for a node.

There are a couple ways we could do this:

a) add a "revision_moderation" field to the node table... not crazy about this idea because it involves "hacking" a core database table.
b) add a new table that just tracks nid and revision_moderation status. Kind of a waste of a table for just one field, but...

I'm going for the second option.