Reduce tracker size by using navigator.sendBeacon

[willfarrell]

Changes

Replace XMLHttpRequest for navigator.sendBeacon. Also adds in the ability to allow events to be triggered on page exit.

This will likely need an update to the back end (sorry, don't know elixir) to change what headers are used (ie use ping-from over Referer maybe?). Not sure what others header are send along with the request that maybe be used by plausible. Some testing will be needed.

It's intended to be used for sending analytics data to a web server, and avoids some of the problems with legacy techniques for sending analytics, such as the use of XMLHttpRequest. ~ MDN

• HTTP on page exit: https://css-tricks.com/send-an-http-request-on-page-exit/
• MDN: https://developer.mozilla.org/en-US/docs/Web/API/Navigator/sendBeacon
• Browser Support: https://caniuse.com/beacon

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Tests

• This PR does not require tests

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Documentation

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Dark mode

• This PR does not change the UI

[ukutaht]

Hi @willfarrell, thanks for the PR!

I think you already read the discussions we've had about this. I think currently Beacon is not well-supported enough to make it the default way of sending pageviews. We have talked about and might incorporate beacon as an optional extra in the future. But we definitely won't use it as the default anytime soon.