Added pluggable Avatar providers

app/User/Avatar/GravatarProvider.php

@@ -0,0 +1,35 @@
+<?php
+
+namespace Kanboard\User\Avatar;
+
+use Kanboard\Core\Base;
+use Kanboard\Core\User\Avatar\AvatarProviderInterface;
+
+class GravatarProvider extends Base implements AvatarProviderInterface
+{
+    /**
+     * Render avatar html
+     *
+     * @access public
+     * @param array $user
+     * @param int   $size
+     */
+    public function render(array $user, $size)
+    {
+        $url = sprintf('https://www.gravatar.com/avatar/%s?s=%d', md5(strtolower($user['email'])), $size);
+        $title = $this->helper->text->e($user['name'] ?: $user['username']);
+        return '<img src="'.$url.'" alt="'.$title.'" title="'.$title.'">';
+    }
+
+    /**
+     * Determine if the provider is active
+     *
+     * @access public
+     * @param  array $user
+     * @return boolean
+     */
+    public function isActive(array $user)
+    {
+        return !empty($user['email']) && $this->config->get('integration_gravatar') == 1;
+    }
+}