Sections: Refactor to allow sections in New Bedford middle schools

[kevinrobinson]

Who is this PR for?

New Bedford students, families, educators

What problem does this PR fix?

New Bedford exports only homeroom assignment data from the SIS, but this doesn't work well for most middle school educators. District folks are updating that separately.

When that is fixed, there are assumptions and configuration and product differences that don't make sense for their MS use case. One example is that in Somerville where sections are used, there is always grade data as well. Another issue is that there's no way to deploy in stages, where we import and validate, then enable for authorization.

What does this PR do?

1. Authorization
   Authorization code now respects PerDistrict#enabled_sections? which can be overridden by an env variable as well.

2. Endpoints
   The section-related endpoints respect the same method.

3. Section links
   This adds another layer, PerDistrict#allow_sections_link?, allowing incremental deployment where we enable the feature but don't make it visible to everyone yet.

4. Student profile UI
   This updates the profile UI to use a different path for New Bedford, where the sections are shown on the profile but without grades like in SHS. This involves changes to LightProfilePage and also pushing code out into PerDistrict.js. EDIT: last commit backs out this actual change for now, to test first

Screenshot (if adding a client-side feature)

Checklists

Which features or pages does this PR touch?

• Navbar
• Student Profile
• Section
• My Sections
• Core

Does this PR use tests to help verify we can deploy these changes quickly and confidently?

• Included specs for changes
• Improved specs for existing code in need of better test coverage
• Manual testing made more sense here

[kevinrobinson]

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