Frame access controls

[mariusandra]

Adds an access key for talking to the frame, plus access controls:

• Private (default): You need a key to both view and control the frame.
• Protected: Everyone can view the frame's image, but you need the access key to update content.
• Public: Everyone can view or control the frame without a key. This makes for the smallest QR codes.

The traffic still goes over unsecured HTTP, so it's not a silver bullet, but a start.

The access key is appended to the URL in the interface and on QR codes when needed