Add config option to exclude fields from auth providers sync

app/Core/User/UserProperty.php

@@ -44,7 +44,7 @@ class UserProperty
      */
     public static function filterProperties(array $profile, array $properties)
     {
-        $excludedProperties = array('username');
+        $excludedProperties = explode_csv_field(EXTERNAL_AUTH_EXCLUDE_FIELDS);
         $values = array();
 
         foreach ($properties as $property => $value) {

app/constants.php

@@ -149,3 +149,6 @@ defined('HTTP_VERIFY_SSL_CERTIFICATE') or define('HTTP_VERIFY_SSL_CERTIFICATE',
 defined('TOTP_ISSUER') or define('TOTP_ISSUER', 'Kanboard');
 
 defined('PROJECT_ACTIVITIES_MAX_EVENTS') or define('PROJECT_ACTIVITIES_MAX_EVENTS', 10000);
+
+// Comma separated list of fields to not synchronize when using external authentication providers
+defined('EXTERNAL_AUTH_EXCLUDE_FIELDS') or define('EXTERNAL_AUTH_EXCLUDE_FIELDS', 'username');