Add user backend provider system

app/Core/User/UserManager.php

@@ -0,0 +1,71 @@
+<?php
+
+namespace Kanboard\Core\User;
+
+/**
+ * User Manager
+ *
+ * @package  Kanboard\Core\User
+ * @author   Frederic Guillot
+ */
+class UserManager
+{
+    /**
+     * List of backend providers
+     *
+     * @access protected
+     * @var array
+     */
+    protected $providers = array();
+
+    /**
+     * Register a new group backend provider
+     *
+     * @access public
+     * @param  UserBackendProviderInterface $provider
+     * @return $this
+     */
+    public function register(UserBackendProviderInterface $provider)
+    {
+        $this->providers[] = $provider;
+        return $this;
+    }
+
+    /**
+     * Find a group from a search query
+     *
+     * @access public
+     * @param  string $input
+     * @return UserProviderInterface[]
+     */
+    public function find($input)
+    {
+        $groups = array();
+
+        foreach ($this->providers as $provider) {
+            $groups = array_merge($groups, $provider->find($input));
+        }
+
+        return $this->removeDuplicates($groups);
+    }
+
+    /**
+     * Remove duplicated users
+     *
+     * @access protected
+     * @param  array $users
+     * @return UserProviderInterface[]
+     */
+    protected function removeDuplicates(array $users)
+    {
+        $result = array();
+
+        foreach ($users as $user) {
+            if (! isset($result[$user->getUsername()])) {
+                $result[$user->getUsername()] = $user;
+            }
+        }
+
+        return array_values($result);
+    }
+}