confluence-space-avatar-favicons: better loading listener

The listener for 'load' event seems to not be doing its job for some
reason, and it might be better to avoid it.  Quote from Stack Overflow:

	"load"

	A very different event, **load**, should only be used to detect
	a *fully-loaded page*. It is an incredibly popular mistake to
	use load where DOMContentLoaded would be much more appropriate,
	so be cautious.

<https://stackoverflow.com/a/36096571/1083697>

Rewrite the loading listener of confluence-space-avatar-favicons.user.js
to use a combination of checking document.readyState and a listener for
DOMContentLoaded event, as per recommendation from Stack Overflow.  This
combination is a recommended replacement for jQuery's $(document).ready
from <https://youmightnotneedjquery.com>.

confluence-space-avatar-favicons.user.js

@@ -7,7 +7,7 @@
 // @include      https://*confluence*/*
 // @match        https://confluence.example.com/*
 // @icon         https://wac-cdn-2.atlassian.com/image/upload/f_auto,q_auto/dam/jcr:5d1374c2-276f-4bca-9ce4-813aba614b7a/confluence-icon-gradient-blue.svg?cdnVersion=691
-// @version      3
+// @version      4
 // @license      MIT
 // @grant        none
 // ==/UserScript==
@@ -21,6 +21,10 @@
 		console.log(LOG_PREFIX, ...toLog);
 	}
 
+	function warn(...toLog) {
+		console.warn(LOG_PREFIX, ...toLog);
+	}
+
 	function changeFavicon() {
 		log("Trying...");
 		const avatar = document.querySelector(".avatar-img") || document.querySelector('img[data-testid="space-icon"]');
@@ -42,7 +46,11 @@
 		}
 	}
 
-	window.addEventListener('load', function() {
+	// https://youmightnotneedjquery.com/#ready
+	if (document.readyState !== 'loading') {
 		changeFavicon();
-	}, false);
+	} else {
+		warn('Cannot load yet. Setting up a listener...');
+		document.addEventListener('DOMContentLoaded', changeFavicon);
+	}
 })();