Fix storybook-build manager-head.html bug

[shilman]

Issue: #1239

What I did

• rename getHeadHtml to getPreviewHeadHtml to make it more explicit
• make sure that we are calling getManagerHeadHtml in both dev and build modes
• chmod +x build.js

How to test

No preview/manager head case:

yarn bootstrap
cd examples/cra-storybook
yarn storybook
yarn build-storybook
cd storybook-public
serve
open http://localhost:5000

Verify that there is no undefined across the top of the screen

Preview/manager head case:

echo "manager" > examples/cra-storybook/.storybook/manager-head.html
echo "preview" > examples/cra-storybook/.storybook/preview-head.html

Run the same tests as above, verify that there is manager across the top of the screen and preview across the top of the preview iframe.

[shilman]

cc @ndelangen @enjoylife

[codecov]

Codecov Report

Merging #1248 into master will increase coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1248      +/-   ##
==========================================
+ Coverage   13.74%   13.75%   +<.01%     
==========================================
  Files         207      207              
  Lines        4633     4632       -1     
  Branches      504      511       +7     
==========================================
  Hits          637      637              
+ Misses       3564     3558       -6     
- Partials      432      437       +5

Impacted Files	Coverage Δ
app/react/src/server/utils.js	32.14% <ø> (ø)	⬆️
app/react/src/server/middleware.js	0% <0%> (ø)	⬆️
app/react/src/server/index.html.js	0% <0%> (ø)	⬆️
app/react/src/server/build.js	0% <0%> (ø)	⬆️
app/react/src/server/iframe.html.js	0% <0%> (ø)	⬆️
addons/knobs/src/components/types/Number.js	8.06% <0%> (ø)	⬆️
...codemod/src/transforms/update-organisation-name.js	40.62% <0%> (ø)	⬆️
addons/info/src/components/PropTable.js	0% <0%> (ø)	⬆️
app/react/src/server/babel_config.js	44.82% <0%> (ø)	⬆️
... and 20 more

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[PavelPolyakov]

Confirming, no undefined there:

[tmeasday]

LGTM and works