automated withdrawal system

[BlinkyStitt]

#28 is more important. But we should work towards having automated withdrawals. We can dump thoughts about it here until we actually have time to code it.

One thing to remember is how this will interact with the referral system. we plan on referrals required at least $10 of credits.

1. user deposits $10 for credits
2. user creates a referral code
3. either:
   a. don't allow them to withdraw their first $10
   b. withdrawing those first $10 disables referrals

[BlinkyStitt]

if a user runs out of credits, their referral code should keep working. That way a referral using our RPC will give enough credits for this user to get out of the free tier.

[Bencyril]

For 3.If announced that withdrawing disables referrals then 3.b is way better, it doesn't let the user thinks he/she's locked
By disabling referral we avoid people abusing the system too

[yenicelik]

I suppose these lines here allow withdrawal: https://github.com/llamanodes/payment-contracts/blob/main/contracts/Sweeper.vy#L40 so we can proceed with implementing the endpoint?

Also according to our last discussion, I suppose we want to proceed with only having a single input token for now? (the smart contract does not need to change I suppose)