Snapshots consolidation

Relies on https://github.com/AshFurrow/Nimble-Snapshots/pull/7 & https://github.com/artsy/eidolon/pull/75

We used to have ReferenceImages everywhere. Now they're in one place. 
Also moved all our snapshots to be named tests.

Would love to write code like this instead:
```
        it("looks right by default") {
            let sut = ConfirmYourBidEnterYourEmailViewController.instantiateFromStoryboard()
            expect(sut) == snapshot("default")
        }
```