Type of `validator_payment_ratio` in struct `ValidatorPrefs`

[readlnh]

Now the type of validator_payment_ratio in ValidatorPrefs is u32, should we change it to Perbill?

[aurexav]

No, I change it in 129416a.

Much easier to understand for front-end. And our prefs is different from substrate at that time.

In #218 it might be refactored.

[hackfisher]

Related #212