[AIP-64] Acquire YFI for participation in the veYFI gauge voting system

Proposal Discussion: https://forum.alchemix.fi/public/d/354-aip-64-acquire-yfi-for-participation-in-the-veyfi-gauge-voting-system

A vote "FOR" authorizes the multisig to purchase $500k of YFI ($250k immediate, $250k over the next 3 months, timing at discretion of multisig), to be locked in veYFI when released and used to vote for yvDAI and yvWETH gauges.

A vote "AGAINST" does not authorize the purchase of any YFI by the Alchemix multisig.