Dry url issue69

This pull request replaces pull/issue #69 as that issue seems to have some work ahead and by my mistake it did not have a dedicated named branch. Now I have made a dedicated branch ``issue69`` to be used to fix everything concerning following problem:

Description:

Urls for contacts, news and vacancies were defined in multiple places: 
* ``urls.py``, 
* ``models.py`` - in ``url_path`` and 
* ``menus.py`` - in ``url_attribute``. 

This situation prevents changing/translating the default urls (i.e. contacts/ news/ etc). In this commit url patterns in ``urls.py`` are given names and then ``reverse`` is used to resolve urls where needed i.e. in  ``Entity.get_related_info_page_url`` or in ``URLModelMixin.get_absolute_url``. 

Url patterns are named so that their name can be deduced from  ``url_path`` and ``url_attribute`` when needed.