Use React and Webpack for kepler

[Rikuoja]

Is your feature request related to a problem? Please describe.
The Kepler.gl UI is not configurable. Also, it is quite fragile.

Describe the solution you'd like
Use Webpack for all JS code, e.g. https://dev.to/icewreck/react-flask-integration-part-1-setup-with-webpack-djo .

Bundle the js needed on other pages separately, too. Use proper React Kepler.gl. We still probably don't want to rewrite all the UI with React, we just need the Kepler view.

The data can be served to Kepler with flask.

[Rikuoja]

Required e.g. for #35

[Rikuoja]

Implementing this requires #51 first.