Add user backend provider system

app/User/DatabaseBackendUserProvider.php

@@ -0,0 +1,43 @@
+<?php
+
+namespace Kanboard\User;
+
+use Kanboard\Core\Base;
+use Kanboard\Core\User\UserBackendProviderInterface;
+use Kanboard\Filter\UserNameFilter;
+use Kanboard\Model\UserModel;
+
+/**
+ * Database Backend User Provider
+ *
+ * @package  Kanboard\User
+ * @author   Frederic Guillot
+ */
+class DatabaseBackendUserProvider extends Base implements UserBackendProviderInterface
+{
+    /**
+     * Find a group from a search query
+     *
+     * @access public
+     * @param  string $input
+     * @return DatabaseUserProvider[]
+     */
+    public function find($input)
+    {
+        $result = array();
+
+        $users = $this->userQuery->withFilter(new UserNameFilter($input))
+            ->getQuery()
+            ->columns(UserModel::TABLE.'.id', UserModel::TABLE.'.username', UserModel::TABLE.'.name')
+            ->eq(UserModel::TABLE.'.is_active', 1)
+            ->asc(UserModel::TABLE.'.name')
+            ->asc(UserModel::TABLE.'.username')
+            ->findAll();
+
+        foreach ($users as $user) {
+            $result[] = new DatabaseUserProvider($user);
+        }
+
+        return $result;
+    }
+}