Update storybook to at least 4.0.8

[sisou]

4.0.8 fixes a bug with knobs, so that the view does not re-render each time a knob is changed.

Since we are still on 4.0.0-alpha.4, lots of things have changed even for the main v4 release, so here is a migration guide:
https://medium.com/storybookjs/migrating-to-storybook-4-c65b19a03d2c

[danimoh]

Maybe a good idea to wait a little longer.
4.0.8+ is still riddled with bugs: storybookjs/storybook#4947
And apparently 5.0 is to be released soon.