Custom invariant implementation

[kentcdodds]

tiny-invariant doesn't include your message in the error it throws in production. I don't agree with this decision. The PR to remove this behavior has been ignored alexreardon/tiny-invariant#172. It's a really simple library so I've implemented it directly in the Epic Stack.

I've also added an invariantResponse which throws a response with a 400 status code. This is appropriate for a lot of use cases we had for invariant so this improves those situations.