Add pubsubhubbub_enabled to default configuration

Rename enable_pubsubhubbub into pubsubhubbub_enabled

See https://github.com/FreshRSS/FreshRSS/issues/865

app/install.php

@@ -229,7 +229,7 @@ function saveStep3() {
 				'prefix' => $_SESSION['bd_prefix'],
 				'pdo_options' => array(),
 			),
-			'enable_pubsubhubbub' => server_is_public($base_url),
+			'pubsubhubbub_enabled' => server_is_public($base_url),
 		);
 
 		@unlink(join_path(DATA_PATH, 'config.php'));	//To avoid access-rights problems

data/config.default.php

@@ -57,6 +57,10 @@ return array(
 	#	SimplePie, which is retrieving RSS feeds via HTTP requests.
 	'simplepie_syslog_enabled' => true,
 
+	# Enable or not support of PubSubHubbub.
+	# /!\ It should NOT be enabled if base_url is not reachable by an external server.
+	'pubsubhubbub_enabled' => false,
+
 	'limits' => array(
 
 		# Duration in seconds of the SimplePie cache,