fix: a bunch of small changes in multiple bridges (#2644)

Mostly refactors.

bridges/GelbooruBridge.php

@@ -68,6 +68,7 @@ class GelbooruBridge extends BridgeAbstract {
 
 	public function collectData(){
 		$content = getContents($this->getFullURI());
+		// $content is empty string
 
 		// Most other Gelbooru-based boorus put their content in the root of
 		// the JSON. This check is here for Bridges that inherit from this one