Removes Babel duplication in Webpack configs

[josemigallas]

What this PR does / why we need it:

Webpacker manages webpack itself, creating config files at compilation time. However this generated configs are being overridden in porta in a hardcoded way that leads to errors and uncontrolled results.

This PR gives this control back to Webpacker and Rails and apply all custom configs appropriately. This way config files look a bit more convoluted but also free us from managing webpack.js files.

Read more about how to manage webpack config with webpacker here.

Thanks to this change we are able to better handle specific setups (dev, test, prod..) as well as reduce duplicated code in a safer and more maintainable way by means of JS.

[codecov]

Codecov Report

Merging #435 into master will increase coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #435      +/-   ##
=========================================
+ Coverage   92.74%   92.8%   +0.05%     
=========================================
  Files        2341    2345       +4     
  Lines       75708   75919     +211     
=========================================
+ Hits        70212   70453     +241     
+ Misses       5496    5466      -30

Impacted Files	Coverage Δ
lib/arel/visitors/oracle12.rb	72.72% <0%> (ø)
spec/acceptance/api/account_plan_spec.rb	100% <0%> (ø)
spec/acceptance/api/account_user_spec.rb	100% <0%> (ø)
spec/acceptance/api/account_spec.rb	100% <0%> (ø)
app/models/account/buyer_methods.rb	93.65% <0%> (+1.58%)	⬆️
lib/api_authentication/by_access_token.rb	95.76% <0%> (+4.23%)	⬆️
config/initializers/access_token_authentication.rb	100% <0%> (+33.33%)	⬆️
config/initializers/oracle.rb	50% <0%> (+48.27%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c145979...c73acfb. Read the comment docs.

[didierofrivia]

I'd say this is the moment to start using https://babeljs.io/docs/en/babel-preset-env, and get rid of es2015, stage-0, etc.

[josemigallas]

@didierofrivia That's the purpose of #394 .. unfortunately it's not working at all, so I thought it would be better to make smaller changes each team, starting with this one.

[damianpm]

It LGTM, what do ou think @didierofrivia ?

[didierofrivia]

Looks good, still circle ci needs to bless this PR.

[hallelujah]

What I am not sure is if the bablerc will be exported in the image. If not we need tonverify that.

[josemigallas]

@hallelujah Please feel free to approve if you think it is enough verification to have the docker-build job