Create admin interface for creating and editing Scenarios

[pbaranay]

The heart of a Scenario is its config, but this is more challenging to construct than it needs to be. An admin interface that allows for easy creation of Scenarios would be a benefit to the project.

I'm currently envisioning a two-part form, where the left part is a dropdown of known fixture names, and the right part is an interface for adding kwargs. It should be possible to easily add/remove rows from the form, like with Django inlines. The ability to rearrange the rows would be a nice touch, too.

The right part should further be composed of two columns to indicate key-value relationships. Like with the "parent" form, it should be easy to add, remove, and rearrange the rows of key-value relationships.