[meta] Create WebAPI for frontend apps

Problem/Motivation

We may eventually need some endpoints so the backend data and logic can be used in mobile apps, React frontend, etc.
One possible benefit is the ability to load entire lessons at once instead of making a request for every single activity which will result in a significant performance boost.

Endpoints:

1. Lesson data with activities
2. Submitting answers for an entire / partial lesson
3. Getting next lesson / activity deltas

Concerns:

1. How the required form elements should be controlled, will an answer plugin ID be sufficient?

Proposed resolution

Possibly it'll be best to use Core's REST API.

Remaining tasks

1. Refine.
2. Create individual tasks for every required endpoint.

User interface changes

None

API changes

Hopefully none except the new APIs

Data model changes

Hopefully none