Add Verifiers deposit in staking mediator and verifiers contracts

[npopeka]

Change specification and add additional functionality to YALLVerification.sol and Staking mediator:
Changes to staking contract:

• inherit new contracts from erc20-staking-contracts;
• add mapping (address => uint256) VerifiersDeposits // verifier address => deposit amount;
• add variable uint256 public MinVerifiersDepositAmount with setter with onlyGovernance modifier. This variable can be set only by governance voting;
• add the addVerifierDeposit ( uint256 _deposit amount). Address with active stake can call this method and increase his vefiersDeposit ;
• add the StakeandAddVerifierDeposit ( uint256 _deposit amount); Address can increase its stake and verifiers deposit;
• add additional check into function unstake(uint256 _amount): unstake amount should be less or equal than stake balance - Verifier Deposit;

Verifiers contract:

• verifiers to submit transaction should have active deposit more or equal to minimum deposit;