Authorization: Centralize section authorization in Authorizer

[kevinrobinson]

Who is this PR for?

K8 educators in New Bedford

What does this PR do?

As we're prepping to release section-based authorization for New Bedford educators, this centralizes all authorization related to Sections into the Authorizer class.

# Example usage:
authorized { Section.all } # authorized
authorized { educator.sections } # assigned

This revises and adds more test cases to verify this all works as expected. Also related to older issue writeups (eg, #1080 (comment)).

Checklists

Which features or pages does this PR touch?

• Home page
• Section
• My Sections
• My Students
• 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]

selfie