move graphie-container css out of the iGraph widget
move graphie-container css out of the iGraph widget

Test Plan: add an image to an interactive graph, check that it is
still positioned behind the interactive graph

Auditors: alex

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