Added pluggable Avatar providers

app/Core/User/Avatar/AvatarProviderInterface.php

@@ -0,0 +1,30 @@
+<?php
+
+namespace Kanboard\Core\User\Avatar;
+
+/**
+ * Avatar Provider Interface
+ *
+ * @package  user
+ * @author   Frederic Guillot
+ */
+interface AvatarProviderInterface
+{
+    /**
+     * Render avatar html
+     *
+     * @access public
+     * @param  array $user
+     * @param  int   $size
+     */
+    public function render(array $user, $size);
+
+    /**
+     * Determine if the provider is active
+     *
+     * @access public
+     * @param  array $user
+     * @return boolean
+     */
+    public function isActive(array $user);
+}