Refactor Measurer (using JsonifyProps and Changeable)
Refactor Measurer (using JsonifyProps and Changeable)

Test Plan: only one thing changed; now when you put in the url, it changes immediately

Auditors: alex

daukantas/perseus,learningequality/perseus,ariabuckles/perseus,iamchenxin/perseus,junyiacademy/perseus,chirilo/perseus,iamchenxin/perseus,sachgits/perseus,huangins/perseus,alexristich/perseus,alexristich/perseus,ariabuckles/perseus,learningequality/perseus,sachgits/perseus,huangins/perseus,junyiacademy/perseus,abe4mvp/perseus,abe4mvp/perseus,ariabuckles/perseus,chirilo/perseus,ariabuckles/perseus,daukantas/perseus