When rejected consent, allow blocked elements to show some indication that it has been blocked

[Original comment](https://github.com/ampproject/amphtml/issues/26735#issuecomment-743055844)

Thanks @SimonHogstromRonbol for this idea.

Currently, when the reject signal is found from ``, we stop building the element. Generally, this means that space the element would occupy is left blank. For amp-img case, the loader will be there and continue to spin endlessly.

This isn't a great UX, it would be better if there was some indication as for why the spinner never stops or the ad isn't shown.

Maybe this would be at the extension level or we could give publishers the ability to customize this.

Could also apply to content blocked by access/subscriptions