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/CustomFields.php

@@ -0,0 +1,60 @@
+<?php
+namespace MailPoet\API\Endpoints;
+
+use \MailPoet\Models\CustomField;
+
+if(!defined('ABSPATH')) exit;
+
+class CustomFields {
+  function __construct() {
+  }
+
+  function getAll() {
+    $collection = CustomField::findMany();
+    $custom_fields = array_map(function($custom_field) {
+      return $custom_field->asArray();
+    }, $collection);
+
+    return $custom_fields;
+  }
+
+  function delete($id) {
+    $custom_field = CustomField::findOne($id);
+    if($custom_field === false or !$custom_field->id()) {
+      return array('result' => false);
+    } else {
+      $custom_field->delete();
+
+      return array(
+        'result' => true,
+        'field' => $custom_field->asArray()
+      );
+    }
+  }
+
+  function save($data = array()) {
+    $custom_field = CustomField::createOrUpdate($data);
+    $errors = $custom_field->getErrors();
+
+    if(!empty($errors)) {
+      return array(
+        'result' => false,
+        'errors' => $errors
+      );
+    } else {
+      return array(
+        'result' => true,
+        'field' => $custom_field->asArray()
+      );
+    }
+  }
+
+  function get($id) {
+    $custom_field = CustomField::findOne($id);
+    if($custom_field === false) {
+      return false;
+    } else {
+      return $custom_field->asArray();
+    }
+  }
+}