Implement TopicController

[BenHenning]

Users will make progress on individual topics based on the corresponding stories and skills completed/practices. There should be a system that easily consolidates and reports this information based on the specific topic and profile IDs. This system also needs to download and maintain the individual topics the user has chosen to keep on the local device.

This system is also responsible for managing all stories, explorations, questions, and skills associated with the topic. These may need to be split into their own systems to simplify implementation.

NB: Since the prototype does not need to support manual downloads, this system needs to, by default, automatically download an entire topic (all skills, concept cards, questions, explorations , etc. corresponding to it).

[BenHenning]

This is blocked by #5, #14, and #6.

[BenHenning]

This includes introducing all of the interfaces for each of the structures contained within a single topic.

[BenHenning]

This also depends on #12.

[BenHenning]

This work may be parallelizable since it's introducing support for multiple lesson structures.

[BenHenning]

Should be done.