metaKey shortcut keys do not seem to work on Chrome/Win10

[no-stack-dub-sack]

Despite our mapping of cmd to metaKey, shortcut keys using cmd do not seem to work on Windows. We may want to look deeper into this at some point since we don't want to ostracize our Windows users from the group of elite "Pairwise Power Users", however, this is a bit difficult to debug/test further without being set up to develop on Windows.

We may be able to test this in a cross browser testing environment, but I'm not sure if key bindings will work (this will be easy to test with a single-key binding like escape on the challenge map).

[bonham000]

I hid the keyboard shortcuts on non-Mac clients until/if we add support for Windows/Linux keyboards.