Part of https://github.com/paritytech/polkadot-sdk/issues/239.
Invariant
1. The number of entries in `Tips` should be equal to `Reasons`.
2. If `OpenTip.finders_fee` is true, then `OpenTip.deposit` should be
greater than zero.
3. Reasons exists for each Tip[`OpenTip.reason`], implying equal length
of storage.
polkadot address: 12zsKEDVcHpKEWb99iFt3xrTCQQXZMu477nJQsTBBrof5k2h
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Co-authored-by: Gonçalo Pestana <g6pestana@gmail.com>
Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
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