feature/hidden-mobile-full-screen

This PR adds a new CSS class that can be used to hide elements on mobile devices (via ```@media``` queries). In particular, this hides some functionality such as "Full Screen" mode that doesn't make sense on a mobile device, since it's already "full screen" to begin with. It also hides admin functionality on mobile devices since mobile is intended primarily for flashcard review, not management.

@jazahn Can you review this?

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[FLASH-135](https://jira.huit.harvard.edu/browse/FLASH-135)