Feature/rewrite plugin webpack

[dingo-d]

Ok so this is quite a big rewrite. In the light of core ticket #40894 I wanted to rewrite the plugin using webpack and more modern ES6 syntax of JS. Plus I wanted to separate business from presentational logic.

It is quite a bit away from your old plain jQuery, but I think that it's understandable enough and that it can encourage others in the WordPress community to start working with modern JS tools :)

I've made several modifications:

• Updated the readme with the build process
• Updated package.json with additional information, and dependencies
• Added ES linter
• Added babel for backward compatibility (needs testing on IE11)
• Cleaned up the code a bit, added proper messages that should be in the ajax response if something is wrong
• Changed POST to GETrequests in the callbacks, since we are not writing anything to the database - it's semantically more correct

To do:

• Replace css with sass in another PR

[dingo-d]

@grappler @ernilambar did you have the time to look at the PR? I would like to start with namespacing the plugin, and maybe adding some unit tests if necessary.

[grappler]

@dingo-d Sorry for the delay. Thank you for working on this.

A couple a of comments and requests

• Could we add the ajaxStopped text to the progress bar?
• When choosing a new theme while a check is running hides the progress bar.

[dingo-d]

@grappler Hi I made the changes you requested, should be better now. I made it so that when you switch the theme the checking will stop. Seems like ok behavior to me.

[dingo-d]

@grappler I fixed all the collisions, I should I try to clean the commit history with interactive rebase?

[dingo-d]

Thanks! I'll get on with other work asap!