Function keys f13 - f24 not supported

[jdsn]

The currently used KeyboardJS library does not allow to use the function keys f13 to f24. Some multimedia keyboards or foot switches make use of them. Using them as puch-to-talk button makes sense to have the rest of the keyboard (and in case of a foot switch: also both hands) free to type is comfortable.

I just created a PR to the KeyboardJS project to add the missing key codes: https://github.com/RobertWHurst/KeyboardJS/pull/150 Once this is merged I would love to see the dependencies of mumble-web be updated to use the the latest KeyboardJS version.

Thanks a lot.