Bind window access if window is defined; add addons channel access too

[tmeasday]

So __STORYBOOK_CLIENT_API__ can be used to get access to the store more directly, especially in
a) contexts without a channel (i.e. JSDOM)
b) places where you want to directly use the render function for the story.

OTOH __STORYBOOK_ADDONS_CHANNEL__ is useful to drive storybook directly via the API that addons use.

Uggh, I would really like to refactor this little snippet of code out into @storybook/core also. I forget why I didn't do it before but I think it was the fact that we were late in the release cycle...

[codecov]

Codecov Report

Merging #3243 into master will decrease coverage by 0.15%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master   #3243      +/-   ##
=========================================
- Coverage   36.06%   35.9%   -0.16%     
=========================================
  Files         437     437              
  Lines        9486    9485       -1     
  Branches     1073     912     -161     
=========================================
- Hits         3421    3406      -15     
- Misses       5389    5500     +111     
+ Partials      676     579      -97

Impacted Files	Coverage Δ
app/vue/src/client/preview/index.js	0% <0%> (ø)	⬆️
app/angular/src/client/preview/index.js	0% <0%> (ø)	⬆️
app/react/src/client/preview/index.js	0% <0%> (ø)	⬆️
app/polymer/src/client/preview/index.js	0% <0%> (ø)	⬆️
app/react/src/server/utils.js	0% <0%> (-83.34%)	⬇️
lib/core/src/server/build-dev.js	0% <0%> (ø)	⬆️
addons/a11y/src/components/Report/Elements.js	0% <0%> (ø)	⬆️
addons/a11y/src/components/Report/Rules.js	0% <0%> (ø)	⬆️
lib/ui/src/modules/api/index.js	0% <0%> (ø)	⬆️
lib/core/src/client/preview/reducer.js	37.5% <0%> (ø)	⬆️
... and 72 more

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