Add onChange prop to EditorPage.
Add onChange prop to EditorPage.

Used for the perseus manifesto.

Test Plan:
Sorry for vagueness. Successfully used this to update perseus one with
an unsaved indicator whenever the question changes.

Auditors: alpert

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