Replace all uses of the background page's require in the popup with messaging

[kzar]

Background

With issue #5080 and #5535 we're switching over to use Webpack instead of our hand-rolled module system. Once we've done that we'll no longer be able to use the require function from the background page in our popup.

In issue #5593 we've begun using messaging to the background page where previously the require function was used directly. We now need to finish that work.

What to change

Change adblockpluschrome/popup.js, adblockpluschrome/notification.js and adblockpluschrome/stats.js to remove all uses of require from the background page. Instead use messaging to communicate with the background page.

[kzar]

We decided to do this as a part of #5593, so closing this as a duplicate.