Implement V5 StakingPool and migrations

[Ivshti]

ETA: must be ready by the end of Feb so we can test it through March

StakingPool

See how enter, leave and withdraw work: https://github.com/AdExNetwork/adex-protocol-eth/blob/v5-aggregate-channels/contracts/StakingPool.sol#L173

It's somewhat similar to the loyalty pool.

UI wise:

• we need partial unbonds
• we need to show individual maturing withdraw requests
• we need a button to rage leave
• restaking button will no longer be shown

Migrations

Migrations will work in the following way: ask the user to migrate, under the hood it does a requestUnbond. When the bond has matured, show another button "finalize migration" that will call unbond on Staking and call the migration contract with the old bond, which will give you the new stake and the reward.

During this time, show the incurred migration reward in the UI.