Improve how we do Flow type checks

[Paulo Ragonha]

Instead of relying on running the type checker manually, we should have it running automatically on every build (or file change in development).

Luckily there is a plugin: https://github.com/happylynx/flow-webpack-plugin

Wdyt @xaviervia. Looks like a pretty neat addition to the Epic v9 release.

[Fernando Via Canel]

We should totally do this.