Refactor form CBV to remove some unnecessary complexity

While creating a CBV to work with inline formsets, I ran into a few places where copy-n-paste was the only way to adjust the behavior that I was after and keep the same feel of the internal Django CBV.  This refactor removes the need to copy-n-paste out of `get` in order to add additional data by moving the generation of the `form` context data into `get_context_data`.

*Note*: I will move this over to Trac.  Opening up here for brief review and comments before becoming official.