Postgres master

**NOT INTENDED FOR MERGE, JUST DISCUSSION**

@jennyd and I are going to sit down and talk this through face-to-face, but other comments (particularly from @issyl0 and @fatbusinessman) should go here. We're aiming for merge tomorrow prior to postgres transition on Tuesday 28th.

`managed_by_transition` shouldn't be here any more - I guess it got back in during a rebase at some point.

I replicated data yesterday from Preview, so now have the transition-postgres database in dev. This is the only diff in the schema.rb I get now when I dump it.