Allow ATokenFlexVoting holders to delegate voting power

[davidlaprade]

The usecase is:

• Alice holds GOV, a governance token
• Alice has delegated her GOV to Bob to vote on her behalf
• Alice hears that she can earn yield on her GOV while still maintaining her voting rights on Aave, b/c they have ATokenFlexVoting
• Alice deposits GOV into Aave and receives aGOV
• Alice still wants Bob to be able to vote on her behalf
• So Alice delegates to Bob through aGOV
• Now when a new proposal is created, Bob can express a voting preference to the atoken with Alice's weight added to his

Thanks to 0xmashroom for the good idea!