Add index.js file for RN 0.49

[alj1s]

Issue:
Since RN 0.49, projects now have a single index.js entry point rather than the platform-specific index.android.js and index.ios.js.

What I did

I have added an index.js file to serve as the entry point for RN 0.49 apps. As there is an existing PR for RN 0.49 ( #1971 ) to update the dependencies, I have only made the change to add the file to the generator templates.

How to test

Link the cli project and run getstorybook.

[alj1s]

I felt this was the simplest solution rather than cater for different versions of RN in the generator script. However, I'd be happy to consider alternatives if you prefer.

[codecov]

Codecov Report

Merging #2176 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #2176   +/-   ##
======================================
  Coverage    21.4%   21.4%           
======================================
  Files         263     263           
  Lines        5812    5812           
  Branches      696     706   +10     
======================================
  Hits         1244    1244           
+ Misses       4033    3996   -37     
- Partials      535     572   +37

Impacted Files	Coverage Δ
.../ui/src/modules/ui/components/layout/dimensions.js	15.62% <0%> (ø)	⬆️
lib/ui/src/modules/ui/containers/left_panel.js	25.71% <0%> (ø)	⬆️
lib/ui/src/modules/ui/configs/handle_routing.js	28.04% <0%> (ø)	⬆️
...ponents/left_panel/stories_tree/tree_decorators.js	33.33% <0%> (ø)	⬆️
addons/knobs/src/components/types/Color.js	8.1% <0%> (ø)	⬆️
addons/info/src/components/Props.js	36.36% <0%> (ø)	⬆️
.../src/manager/containers/CommentsPanel/dataStore.js	34.97% <0%> (ø)	⬆️
lib/ui/src/modules/ui/libs/filters.js	47.36% <0%> (ø)	⬆️
lib/ui/src/modules/shortcuts/actions/shortcuts.js	6.25% <0%> (ø)	⬆️
...rc/modules/ui/components/left_panel/text_filter.js	30.98% <0%> (ø)	⬆️
... and 12 more

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[danielduan]

@ximenean thanks for this!

I'm not sure how to proceed with this PR. I think merging this to master directly would help newcomers. This doesn't benefit an existing user who upgraded to 0.49 so the user has to be knowledgeable enough to figure out to add that index file.

Maybe that's not our responsibility if they upgraded their own dependencies.

[alj1s]

@danielduan - I think that just adding something to the docs for RN 0.49 would suffice. I'm not sure I like any alternatives I can think of to automatically handle the upgrade from the storybook side.

If you think there's a good solution to this, I'm happy to have a go at it.