ATokenYieldSource save gas with pre-approval

[code423n4]

Handle

cmichel

Vulnerability details

ATokenYieldSource should approve the lending contract once during initialization with the max value.
This saves gas on every supplyTokenTo/_depositToAave call as the approval can be removed from there.

[asselstine]

We considered this, but it's possible for a malicious user to "drain" the approval of the contract, so there would need to be checks to see if approval dropped below a certain level. We opted to leave out the complexity.

[asselstine]

Actually, we'll tackle this. We will:

1. approve max on init
2. provide a function to approve max again

[PierrickGT]

Add the function to approve max again to the Idle yield source: https://github.com/code-423n4/2021-06-pooltogether/blob/85f8d044e7e46b7a3c64465dcd5dffa9d70e4a3e/contracts/yield-source/IdleYieldSource.sol#L65