Optimize @storybook/addon-storyshots peerDependencies

[Hypnosphi]

Issue: #2303

What I did

Moved babel-core and @storybook/channels to direct dependencies. Removed @storybook/react peer dependency as storyshots may be used with @storybook/react-native as well

How to test

Change babel-core and @storybook/channels in addons/storyshots/package.json to some earlier versions (without caret), like this:

"@storybook/channels": "3.2.15",
"babel-core": "6.25.0",

This way @storybook/addon-storyshots will get its own instances of those packages.

Run

yarn bootstrap --core
yarn test --core # this one uses storyshots for cra-kitchen-sink

[codecov]

Codecov Report

Merging #2337 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2337   +/-   ##
=======================================
  Coverage   16.64%   16.64%           
=======================================
  Files         302      302           
  Lines        7769     7769           
  Branches      778      779    +1     
=======================================
  Hits         1293     1293           
+ Misses       5861     5836   -25     
- Partials      615      640   +25

Impacted Files	Coverage Δ
lib/ui/src/modules/ui/configs/init_panels.js	25% <0%> (ø)	⬆️
addons/info/src/components/Story.js	30.8% <0%> (ø)	⬆️
addons/jest/src/components/Result.js	0% <0%> (ø)	⬆️
addons/knobs/src/components/PropField.js	10.63% <0%> (ø)	⬆️
addons/a11y/src/components/Report/Info.js	0% <0%> (ø)	⬆️
addons/info/src/components/markdown/code.js	24.13% <0%> (ø)	⬆️
...t-native/src/preview/components/StoryView/index.js	0% <0%> (ø)	⬆️
addons/info/src/components/Props.js	37.2% <0%> (ø)	⬆️
addons/knobs/src/components/types/Date/index.js	23.52% <0%> (ø)	⬆️
lib/ui/src/modules/ui/routes.js	0% <0%> (ø)	⬆️
... and 36 more

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