Add device argument for UHK demos

[mondalaci]

Instead of linking the https://ultimatehackingkeyboard.github.io/agent/ URL from both the UHK 60 and UHK 80 landing pages, I want to display relevant keyboards in the Agent demo, so I'd use the:

• https://ultimatehackingkeyboard.github.io/agent?device=uhk60 and
• https://ultimatehackingkeyboard.github.io/agent?device=uhk80 get parameters for Agent to display the relevant keyboard.