Replace jQuery by vanilla JS

@lrwebks thinks it's too heavy for him :)

@lrwebks it's not that hard as you think, please review and test manually by time.

Guess it's a great chance to learn how it works (and be less in fear then for the future :))

PS: My implementation is untested, so please review and try carefully!

@grevil: Please review code-wise.
@lrwebks please test if this works as expected and nothing is broken

@anybody: Tested it with multiple different ad types and it works as expected! Even the problem I had personally, where the ads would only display for regular users and not admins, seems to have resolved itself. That leads me to believe it might actually have been a JQuery quirk as well… Who knows?

Re #4 by the way: The solution you have implemented is totally understandable to me and I can definitely grasp the fetch API. But converting that myself would have certainly wasted more worker hours than it should! So, glad that someone else took care of it.

Code-wise, this also looks good to me!