feat: Backfill and Django Admin for Learning Sequence Outline

* Adds the backfill_course_outlines management command to contentstore
* Adds a read-only Django admin interface to learning_sequences for the
  support team and debugging.
* Adds two new functions to the learning_sequences public API:
  key_supports_outlines and get_course_keys_with_outlines

The learning_sequences app isn't supposed to know about contentstore or
modulestore, as it's intended to be extracted out of edx-platform in the
long term. Therefore, the backfill_course_outlines command is in
contentstore, and not learning_sequences.

This work was tracked in TNL-7983, but it also fixes a bug where we were
trying to generate course outlines for libraries (TNL-7981).

All Open edX instances upgrading to Lilac should run the
backfill_course_outlines command as part of their upgrade process.