Stop requiring our own babel-loader

[kumar303]

Storybook (as of https://github.com/mozilla/addons-code-manager/pull/65) needs babel-loader but this potentially conflicts with create-react-app. We have to specify our own babel-loader for now but we can remove that once https://github.com/storybooks/storybook/issues/5183 is fixed. If we continue to specify our own babel-loader, we may run into future incompatibilities with create-react-app.

┆Issue is synchronized with this Jira Task

[willdurand]

Looks like this will be solved in the next upcoming stable Storybook release.