Do not respond to right alt (Alt Gr) keyboard events.

[q3k]

On some keyboard layouts (eg. pl) the right alt key (a.k.a. Alt Gr) is used in combination with letters to type special characters, eg. AltGr-C makes a 'ć' on a pl keyboard layout.

At least on Firefox on macOS it seems like events that would otherwise result in a special character still get passed to keydown and friends. This means that if etherpad handles alt-c, it might swallow someone's intent to type in 'ć' and instead open up the chat window.

This could arguably be a bug in Firefox or macOS, as the same doesn't happen in Firefox on Linux/X11. But before I bring the discussion to a wider audience I'd like to at least make the issue go away at etherpad level.