CKEditor: Figure out a way to clear its cache so that plugin changes show up right away.

This came up while working on #3384

@Graham-72:

... I found that once I had made sure that my browser was using the revised plugin script (by forcing it to clear its cache and hard reload) I was no longer getting the TypeError: Argument 1 ... error report.

@quicksketch:

... Yeah a problem with CKEditor is that it loads the JavaScript via it's own mechanism AND it's inside of an iframe, so plugin changes don't show up right away. Seems like something we might want to try and figure out if we can cache-bust based on Backdrop version numbers or something, to prevent this problem for end-users that update.