Allow specifying left/right Control key for hotkeys

Some of my friends used to bind Right Ctrl to Mute on Discord, which leaves Left Ctrl free for all the things Ctrl keys do. On Fluxer, the left and right modifier keys are treated identically, and Left Ctrl isn't as convenient a hotkey.

When making keybinds for modifier keys, specify the Left or Right version of the modifier key. If someone really wants both versions, they can make two keybinds.

As mentioned, this works on Discord. Interestingly, Discord labels Left Ctrl as just Ctrl, even though the hotkey then doesn't respond to Right Ctrl. It does label Right Ctrl as Right Ctrl.