Fix/suppress current eslint warnings

[Hypnosphi]

This also raises import/no-unresolved and import/extensions to error level
everywhere except docs.

I didn't touch the warning about lacking tests for addons/notes so that it keeps reminding that they still have to be written.

It would be nice if we could find a way to know if some new warnings are introduced with a PR (like it's done with code coverage). Does anyone know how to get this metric from CircleCI?

[codecov]

Codecov Report

Merging #1936 into master will decrease coverage by 0.05%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1936      +/-   ##
==========================================
- Coverage   21.43%   21.38%   -0.06%     
==========================================
  Files         261      262       +1     
  Lines        5743     5757      +14     
  Branches      691      689       -2     
==========================================
  Hits         1231     1231              
- Misses       3987     4010      +23     
+ Partials      525      516       -9

Impacted Files	Coverage Δ
addons/graphql/demo/index.js	0% <ø> (ø)	⬆️
lib/ui/example/server.js	0% <ø> (ø)	⬆️
app/react-native/src/bin/storybook-start.js	0% <0%> (ø)	⬆️
addons/notes/src/register.js	0% <0%> (ø)	⬆️
addons/graphql/demo/.eslintrc.js	0% <0%> (ø)
app/react/src/server/config/babel.js	0% <0%> (-100%)	⬇️
app/react/src/server/babel_config.js	0% <0%> (-77.42%)	⬇️
addons/knobs/src/components/types/Number.js	8.06% <0%> (ø)	⬆️
addons/storyshots/src/require_context.js	0% <0%> (ø)	⬆️
...ponents/left_panel/stories_tree/tree_decorators.js	33.33% <0%> (ø)	⬆️
... and 23 more

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[usulpro]

👍

[igor-dv]

Thinking out loud - can we maybe create a separate CI job for the linting that will be run with the error level for every rule. This job will not prevent anything from merging, but just illustrate the worst picture of the PR.

[Hypnosphi]

a separate CI job for the linting that will be run with the error level for every rule

The problem is it would be red for all subsequent PRs even if they don't introduce new warnings, so it wouldn't really give much value