Referral program for Lido stakers.

Propose to start referral program for Lido stakers.

1. Referrer to receive 15 LDO for every 1 ETH staked to Lido (for txs of 1 ETH minimal)
2. The total referral rewards pool to be set to 15 mln LDO
3. At any point, DAO can stop the program or change the amount of LDO for the rewards pool
4. The payouts are to be calculated in two-week periods & paid out the week after the period's end

The program was discussed on the forum: https://research.lido.fi/t/lido-referral-program/715