Adding the challenge scaffold

First PR for https://github.com/ijustwanttocode/whyeatthings/issues/43
- Create the new challenge (scaffold created and modified)
- There are a few empty files here but I plan to use them almost immediately
- No authentication/authorization just yet

Other:
- Updated the scaffold template for `_form.html.slim` to use simple_form.
- modified the config/intializers/assets.rb file.