Allow rewards to be batch claimed onBehalf

[jcmonte]

Auditor Recommendation

It is regarded as best practice to allow keeper bots to claim rewards on users' behalf. This will allow users to receive their rewards even when they are inactive for a short period. This could be achieved by adding
a function similar to the following to the StakingRewards contract:

function getRewardOnBehalf(address[] stakers) external returns (bool) {
    for (uint i; i < stakers.length; i++) {
        _updateRewards(stakers[i]);
        uint256 reward = rewards[stakers[i]];
        if (reward > 0) {
      rewards[stakers[i]] = 0;
      // Send the rewards to Escrow for 1 year
      stakingToken.transfer(address(rewardEscrow), reward);
      rewardEscrow.appendVestingEntry(stakers[i], reward, 52 weeks);
      emit RewardPaid(stakers[i], reward);
    }
}

By allowing a keeper bot to claim multiple users' rewards in a single transaction, the overall gas cost can be decreased to the gas refunds given for reaccessing storage slots.