Add support for multiple webpack chunks in iframe

[ndelangen]

This is a replacement PR for #677

This allows an arbitrary number of files to be loaded into the iframe,
not just the single preview.bundle.js file.

Webpack plugins can output chunks in a separate file. In dev, since
file names are predictable, I can add the js file into a head.html file.
When building storybook statically, however, js files have a hash added
to them, so this doesn't work.

This change makes the static build parse all webpack chunks and load
them in the iframe (excluding the manager script), instead of only
including known files.

[ndelangen]

@apexskier Can you please review and test?

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@bce5716). Click here to learn what that means.
The diff coverage is 0%.

@@            Coverage Diff            @@
##             master    #1083   +/-   ##
=========================================
  Coverage          ?   13.64%           
=========================================
  Files             ?      207           
  Lines             ?     4640           
  Branches          ?      517           
=========================================
  Hits              ?      633           
  Misses            ?     3555           
  Partials          ?      452

Impacted Files	Coverage Δ
app/react/src/server/config/webpack.config.js	0% <ø> (ø)
app/react/src/server/middleware.js	0% <0%> (ø)
app/react/src/server/iframe.html.js	0% <0%> (ø)
app/react/src/server/index.js	0% <0%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update bce5716...207eb44. Read the comment docs.

[ndelangen]

• I've extended the original PR to also work for devmode.
• The message "storybook started" has been moved till after webpack has compiled.
• I've added a progress indicator whilst webpack is busy.

Can someone please review?