Some custom answer types return new-style objects

Fixes #78429.

Because they just call into the other validators directly.

Not thrilled about this change but I guess working is better than not working.

As for why linear_equation_word_problems uses a custom validator for grading, I'm not sure.

Test Plan:
Answered linear_equation_word_problems.html?debug&problem=magazine_subscription_selling&seed=23.

Auditors: eater, alex