Make it possible to opt-in for I2P in the Greeter

The short-term fix (#7722 (closed)) regarding I2P requires users who want to enable I2P to edit the kernel command-line. Once we have a revamped Greeter, it would be good to move this option there.

Related issues

Has duplicate #9512 (closed)
Blocked by #7722 (closed)

Original created by @intrigeri on 7723 (Redmine)

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