Implement rate limiting via API Gateway

[pfych]

API Gateway likely offers a method of rate limiting, if we can do it via our serverless.yml its 10x better than doing it in our lambda since that's where we pay money for execution time.

[pfych]

This might have to be done with AWS WAF.

https://docs.aws.amazon.com/waf/latest/APIReference/API_RateBasedStatement.html

Potentially do a cost comparison of someone pummelling the instance vs implementing WAF rules, seems like it can range $1/$5 fixed cost monthly.

[pfych]

WAF might be overkill: https://docs.aws.amazon.com/apigateway/latest/developerguide/api-gateway-request-throttling.html

[pfych]

This will be resolved once #5 is merged