Section authorization: Optimize method to make it faster

[kevinrobinson]

Who is this PR for?

Educators with section assignments

What problem does this PR fix?

#2654 centralizes section authorization, and makes more use of these methods that previously. This slowed down the section page for queries like authorized { Section.all }.

What does this PR do?

Adds two optimizations, adapted from #why_authorized_for_student?, comparing foreign key values rather than model values to remove that query, and adding a #to_a call to the end of the association, which causes Rails to cache the result of that query across calls. I'm not sure why this influences Rails' caching, but it does and PerfTests here show the same thing as with #why_authorized_for_student?.

Also adds minimal smoke tests to PerfTest methods, just verifying that they still run and don't break. It does not make assertions about performance.

Screenshot (if adding a client-side feature)

puts PerfTest.section_authorization_pattern(0.01);nil

# before
1%,  median: 1917ms    p95: 2490ms

# after
 1%, median:  417ms    p95:  613ms
 5%, median:    2ms    p95:  511ms
10%, median:    2ms    p95:  518ms
(caveat: distribution is bimodal since some educators don't have sections)

Checklists

Which features or pages does this PR touch?

• Section
• Authorization

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

• Manual testing made more sense here

[kevinrobinson]

selfie