Update Netlify CMS plugin to replace webpack loader exclusions

[erquhart]

#4046 will revert the changes introduced in #3611. The purposes of the commit being reverted as mentioned in the PR:

• uses arrays when defining webpack loader.exclude so that exclusions may be added by plugins
• adds node_modules/netlify-cms to css loader exclusions (build-javascript stage)

Since the exclusions will go back to being strings, we should be able to address this by passing a function to the webpack-configurator loader method to replace the string with an array and achieve the same result.

@KyleAMathews you can consider this assigned to me.

[KyleAMathews]

👍

[KyleAMathews]

Added you as a collaborator so you can assign stuff to yourself directly :-)