Fixes T3422 - imageModal positioning for smaller screens

Summary: The imageModal was getting cut off on smaller screens, so I've made it always be centered for full visibility on the largest range of screen sizes

Test Plan:
- Open imageModal
- Resize window
- See modal stay centered

Reviewers: john, brian, pamela

Reviewed By: pamela

Subscribers: #cs

Differential Revision: https://phabricator.khanacademy.org/D14856