Implement persistent storage

[tomfa]

Storage is planned to be a simple, generic key-value storage that returns json. /api/data/<id> should return the (json) value from the key <id>.

Alternatives:

• S3, where key = "filename" and value is its content
• Firebase.

@andrroy and me talked briefly about it yesterday. We're both noobs in Firebase, and want to learn. So we're leaning towards that.

There's dummy storage code in api/data/[id].js.

[tomfa]

This gist can be related if we choose aws s3 storage
https://gist.github.com/tomfa/7bb519a34262353087a83712539eb6b0