Added pluggable Avatar providers

app/Core/User/Avatar/AvatarManager.php

@@ -0,0 +1,62 @@
+<?php
+
+namespace Kanboard\Core\User\Avatar;
+
+/**
+ * Avatar Manager
+ *
+ * @package  user
+ * @author   Frederic Guillot
+ */
+class AvatarManager
+{
+    /**
+     * Providers
+     *
+     * @access private
+     * @var AvatarProviderInterface[]
+     */
+    private $providers = array();
+
+    /**
+     * Register a new Avatar provider
+     *
+     * @access public
+     * @param  AvatarProviderInterface $provider
+     * @return $this
+     */
+    public function register(AvatarProviderInterface $provider)
+    {
+        $this->providers[] = $provider;
+        return $this;
+    }
+
+    /**
+     * Render avatar html element
+     *
+     * @access public
+     * @param  string   $user_id
+     * @param  string   $username
+     * @param  string   $name
+     * @param  string   $email
+     * @param  int      $size
+     * @return string
+     */
+    public function render($user_id, $username, $name, $email, $size)
+    {
+        $user = array(
+            'id' => $user_id,
+            'username' => $username,
+            'name' => $name,
+            'email' => $email,
+        );
+
+        foreach ($this->providers as $provider) {
+            if ($provider->isActive($user)) {
+                return $provider->render($user, $size);
+            }
+        }
+
+        return '';
+    }
+}