Consider removing flow-type (for now)

[phyrog]

We haven't really used flow in the code up until now, and seeing the lagging library support for flow (at least in Apollo, which is quite critical for us), we might as well remove it for now.

Later on it might be good to reconsider using Typescript, as it seems to have the more mature ecosystem (given create-react-app will then support it, see facebook/create-react-app#2815).

[phyrog]

Closed by #173