Review: Imbalance handling in runtime

[hackfisher]

type RingReward: OnUnbalanced<PositiveImbalanceRing<Self>>;
	/// Handler for the unbalanced increment when rewarding a staker.
	type KtonReward: OnUnbalanced<PositiveImbalanceKton<Self>>;

[hackfisher]

https://substrate.dev/rustdocs/master/frame_support/traits/trait.Imbalance.html

Post documents moved to wiki: https://github.com/darwinia-network/darwinia/wiki/Best-Practice-for-Darwinia-Security-Review

[aurexav]

Move to #353.