Allow dot in user name

0f273988 changed the regular expression for user names. The new regex
does not allow dots anymore in user names, stripping them silently.
We already have at least one user with a dot in their name who can't
login now anymore.

includes/model/User_model.php

@@ -273,7 +273,7 @@ function User_ids()
  */
 function User_validate_Nick($nick)
 {
-    return preg_replace('/([^\p{L}\p{N}-_ ]+)/ui', '', trim($nick));
+    return preg_replace('/([^\p{L}\p{N}-_. ]+)/ui', '', trim($nick));
 }
 
 /**