Login Module - Uses current URL in the action attribute of the form element

**Description**

Problem is described here: https://github.com/joomla/joomla-cms/issues/5345

**How to test**

Try to login and logout on several pages of the Joomla! installation, especially on pages without menu entry (e.g. category of an article without menu entry), all processes should work properly. To see the change, open the source code of such a page and compare the URL in the action attribute of the form.