feat: first attempt at course outline changes

[michaelroytman]

Description

Describe what this pull request changes, and why. Include implications for people using this change.
Design decisions and their rationales should be documented in the repo (docstring / ADR), per
OEP-19, and can be
linked here.

Useful information to include:

• Which edX user roles will this change impact? Common user roles are "Learner", "Course Author",
  "Developer", and "Operator".
• Include screenshots for changes to the UI (ideally, both "before" and "after" screenshots, if applicable).
• Provide links to the description of corresponding configuration changes. Remember to correctly annotate these
  changes.

Supporting information

Link to other information about the change, such as Jira issues, GitHub issues, or Discourse discussions.
Be sure to check they are publicly readable, or if not, repeat the information here.

Testing instructions

Please provide detailed step-by-step instructions for testing this change.

Deadline

"None" if there's no rush, or provide a specific date or event (and reason) if there is one.

Other information

Include anything else that will help reviewers and consumers understand the change.

• Does this change depend on other changes elsewhere?
• Any special concerns or limitations? For example: deprecations, migrations, security, or accessibility.
• If your database migration can't be rolled back easily.

[michaelroytman]

As a result of these changes, if the feature flag is enabled,

• we will NOT remove usage keys from the course outline if the learner is an audit learner and the usage key is partitioned to verified learners
• we WILL mark such usage keys as "inaccessible", which will be exposed via the API

As a result, usage keys that would ordinarily be hidden from the outline will be included in the API response. Of these course blocks, the ones that should appear "locked" are those that are NOT in the accessible_sequences list in the API response.