Create learning_sequences app, Course Outline API.

Introduces the learning_sequences app, intended to provide metadata for
sequences and course outlines. The short term goal is to provide faster
source of this information for the new Courseware microfrontend
(frontend-app-learning). The medium term goal is to provide an in-proc
API that is useful to other parts of the platform that need fast access
to course outline information customized for a user. The long term
goals are outlined in the README.rst.

This first iteration of the API only lays out the basic structure for
how we'd arrange the pieces, with enough of an implementation to feel
"real" (simple schedules, staff_only content hiding). It's not ready to
be turned on, and is not currently plugged into the publish-flow. The
only way to get data into this new API is via the update_course_outline
management command. The REST endpoint is also currently limited to
global staff only, though it's possible to get the outline for a student
by using the ?username= query parameter.

TNL-7122