#6496 n plus one cases cases index

[andoq]

What github issue is this PR for, if any?

Resolves #6496

What changed, and why?

• use .size instead of .count, eager load the casa_case_emancipation_categories in the controller
• memoize emancipation category count
  • NOTE: this wasn't strictly part of the ticket, and is probably a trivial cost do do a COUNT(*) on this table since there are a limited number of categories, but was easy to fix and covered in all the same testing, so I added it.

How is this tested? (please write rspec and jest tests!) 💖💪

• ran all tests, but specifically:
  • emancipations_helper_spec
  • casa_cases_decorator_spec

Ran and loaded casa_cases_index page (/casa_cases) as a voluteer and as a supervisor
Ran and loaded case_cases_show page (/casa_cases/cina-10-1002) as a volunteer and as a supervisor.

Screenshots please :)

Here are screens of the server development log and queries before and after the change. Annotated where the queries are changed.

Before fix	After fix