Added API/Endpoint abstract class

- (re)Added Endpoints folder to both API and Router
- fixed syntax in namespaces
- xhr.responseJSON is returned to the fail()
- fixed Router endpoints (view in browser, cron,...)

lib/API/Endpoints/NewsletterTemplates.php

@@ -0,0 +1,41 @@
+<?php
+namespace MailPoet\API\Endpoints;
+
+use MailPoet\Models\NewsletterTemplate;
+
+if(!defined('ABSPATH')) exit;
+
+class NewsletterTemplates {
+  function __construct() {
+  }
+
+  function get($id = false) {
+    $template = NewsletterTemplate::findOne($id);
+    if($template === false) {
+      return false;
+    } else {
+      return $template->asArray();
+    }
+  }
+
+  function getAll() {
+    $collection = NewsletterTemplate::findMany();
+    return array_map(function($item) {
+      return $item->asArray();
+    }, $collection);
+  }
+
+  function save($data = array()) {
+    $template = NewsletterTemplate::createOrUpdate($data);
+    return ($template->getErrors() === false && $template->id() > 0);
+  }
+
+  function delete($id) {
+    $template = NewsletterTemplate::findOne($id);
+    if($template !== false) {
+      return $template->delete();
+    } else {
+      return false;
+    }
+  }
+}