Improve BBCode:toPlaintext

- Fix issue where matching literal square brackets were removed with their content
- Fix issue where content without line feeds between BBCode tags would end up compacted in plain text
- Update extr calls to BBCode::toPlaintext in api

src/Content/Text/BBCode.php

@@ -357,10 +357,7 @@ class BBCode extends BaseObject
 	 */
 	public static function toPlaintext($text, $keep_urls = true)
 	{
-		$naked_text = preg_replace('/\[.+?\]/','', $text);
-		if (!$keep_urls) {
-			$naked_text = preg_replace('#https?\://[^\s<]+[^\s\.\)]#i', '', $naked_text);
-		}
+		$naked_text = HTML::toPlaintext(BBCode::convert($text, false, 0, true), 0, !$keep_urls);
 
 		return $naked_text;
 	}