fix: googletag.display errors when ad slot not in DOM

[caffeinated-pixels]

Description

This PR fixes an issue where GPT complains if googletag.displaySlots() is called for a defined ad slot that is not yet in the DOM, i.e.

GPT] Error in googletag.display: could not find div with id "my-ad-id" in DOM for slot: /my/ad/unit/path

• The fix is to defer the calling of Advertising.displaySlots(). So instead calling this method in Advertising.setupGpt(), we are now calling it at the end of Advertising.active().
• I've also refactored displaySlots() to accept an id and to only call window.googletag.display(id) for this single id, instead of iterating through all the slots
• Advertising.active() gets called from the AdvertisingSlot component, so now Advertising.displaySlots() will not fire for defined ad slots that are not yet in the DOM.
• The solution was based on a suggestion from @Trav84, as outlined in: https://github.com/orgs/KijijiCA/discussions/141

[rcedev]

Is there a reason this hasn't been merged? I'm getting the same console error messages, where GPT is attempting to display an ad before the div element has been rendered to the DOM.

[caffeinated-pixels]

Is there a reason this hasn't been merged? I'm getting the same console error messages, where GPT is attempting to display an ad before the div element has been rendered to the DOM.

We were waiting on @Trav84 to test locally, but didn't hear back, so it kinda got forgotten about. From my own local testing, it seemed to work as expected but it would be good if someone else was also able to test.