Added constants.js file to core

@garrensmith, @rkowalski, @ocelotpotpie - could I get a bit of feedback on this ticket: 

- Is FauxtonAPI.constants the place for this? I think an app-wide constants file is certainly needed & belongs in app/core, but perhaps it should just include it as a dependency for any file needing it rather than adding it to FauxtonAPI? It would be less verbose to use, e.g.

```javascript
define(["constants"], function(C) {
	// C.MY_CONSTANT 
});
```

instead of `FauxtonAPI.constants.MY_CONSTANT`.

Thoughts? 

- Uppercase for constants OK? (cribbed from Google’s JS style guide).

By the way, for now I just added a single constant and updated a single file to use it. From here on out I’ll start using it for future work.