Safari: File system view context menu does not work on iOS devices

[ianyong]

This affects all browsers (since they're all just WebKit under the hood) on iOS devices. See #2242 (review) for more information.

[ianyong]

I managed to get hold of an old iPhone for testing. It seems that Safari on iOS doesn't even fire the context menu event. Unfortunately, it seems that Safari on iOS straight up does not support context menus.

[ianyong]

Not working on this issue because the Source Academy does not officially support Safari (see #2242 (comment)). Leaving this issue open for reference should support be added in the future.