Refactor all startup code into @storybook/core also

[tmeasday]

I'm not sure why we didn't do this already @shilman? It seems an obvious thing to do.

It's a little unfortunate that RN is so different in this respect. Still I am happier making any changes in 2 places (core + RN) than 5+

[tmeasday]

PS I suspect the branch name is incorrect here. This is unlikely to be the final refactor.

[igor-dv]

Oh, I am ATM doing the same for a config part 😱.

[codecov]

Codecov Report

Merging #3259 into master will increase coverage by 0.59%.
The diff coverage is 57.69%.

@@            Coverage Diff             @@
##           master    #3259      +/-   ##
==========================================
+ Coverage   35.61%   36.21%   +0.59%     
==========================================
  Files         442      443       +1     
  Lines        9777     9737      -40     
  Branches      897      909      +12     
==========================================
+ Hits         3482     3526      +44     
+ Misses       5704     5638      -66     
+ Partials      591      573      -18

Impacted Files	Coverage Δ
app/polymer/src/client/preview/index.js	0% <0%> (ø)	⬆️
app/react/src/client/preview/index.js	0% <0%> (ø)	⬆️
app/vue/src/client/preview/index.js	0% <0%> (ø)	⬆️
app/angular/src/client/preview/index.js	0% <0%> (ø)	⬆️
lib/core/src/client/preview/index.js	0% <0%> (ø)	⬆️
lib/core/src/client/preview/start.js	65.67% <69.76%> (ø)
addons/info/src/components/types/Enum.js	0% <0%> (ø)	⬆️
lib/core/src/server/build-dev.js	0% <0%> (ø)	⬆️
...ns/viewport/src/manager/components/viewportInfo.js	33.33% <0%> (ø)	⬆️
lib/core/src/client/manager/index.js	0% <0%> (ø)	⬆️
... and 70 more

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