Fix user configuration in extension (#3412)

Before, only one extension could be configured at a time. Thus we
were loosing the configuration for other extensions when saving.
Now, each extension can be saved without overriding data.

See #3397

lib/Minz/Extension.php

@@ -240,7 +240,7 @@ abstract class Minz_Extension {
 		if (!$this->isUserConfigurationEnabled()) {
 			return;
 		}
-		if ($this->isExtensionConfigured()) {
+		if (FreshRSS_Context::$user_conf->hasParam($this->config_key)) {
 			$extensions = FreshRSS_Context::$user_conf->{$this->config_key};
 		} else {
 			$extensions = [];