Analytics: Change load event to fire from the window instead of created script element

[anselmbradford]

We have this analytics code that fires a gtmloaded event when a script tag has loaded. The cfpb-analytics package looks for this event (see https://github.com/cfpb/cfpb-analytics/blob/main/packages/cfpb-analytics/src/cfpb-analytics.js#L109).

If the gtm script loads before other scripts that use cfpb-analytics, it'll fire the gtmloaded event before other scripts. This PR changes this so that the load event fires on the window, so that it'll fire after all page scripts have loaded.

Changes

• Change load event listening from gtm script tag to window object.

How to test this PR

1. This would need to be merged into production and then pages like http://localhost:8000/about-us/blog/?debug-gtm=true should show debug messages in the console when scrolling the page.

[anselmbradford]

Turns out this seems to be an issue in Chrome, as the logging is working fine in Safari. Closing in favor of #8018. We might need to revisit this if in fact Chrome isn't logging at all.