AWS Lambda support

Description

AWS Lambdas are one of the best serverless options, and they include a very nice free tier.

User Stories

As a Frontity developer
I want to use the Frontity server in AWS Lambdas
so that I can benefit from cheap serverless

Possible solution

We could use this library https://github.com/dougmoscrop/serverless-http but I think I’m more in favor of using the official Express library directly (https://github.com/awslabs/aws-serverless-express) because it has better support. How to use it with Koa is explained in this other library: https://github.com/compwright/aws-serverless-koa/blob/master/index.js