Allow overriding of challenge and group behaviours for better customisation

[trickeydan]

This PR moves much of the inner functionality of challenges to functions that can be overridden when creating a class, similar to Django class based views.

This enables us to dynamically generate flags, content and solutions depending on request context.

This might be particularly helpful for CTFs that wish to store data for the user session.

This introduces the following notable changes:

• Breaking Change: challenges are now instantiated when registered to a group
• Breaking Change: group is now passed as a constructor argument to a challenge
• Deprecation of introduction_md and introduction_html in favour of parts

[Skyler84]

This all looks really cool!