Implement faker randomization with challenge render and answer checker

[fpayer]

• Save the full, raw YAML in the Challenge model so that it can be used for rendering.
• Update the Challenge.render function to call each of it's question's .render functions.
• Create a Question.render function to render the question using the templating engine. Use the SEED_FORMAT_STRING in the Question model to seed the faker instance.
• Implement the Question.check_answer function