rename community/page.html to community/base.html in templates
create view mixin to add context to Community menu + unittests
use CommunityMenuMixin for CommunityPageView
change CommunityMenuMixin

* make it to accept a page slug instead of CommunityPage object
* now it can accept an arbitrary string, like 'news' or 'resources'
* add unittest to this usecase
add docstrings to CommunityMenuMixin unittest and fix typo
refactor CommunityPageView according to mixin changes

* remove additional SQL query
* change get_page_slug()
create UserDetailsMixin + unittests
use UserDetailsMixin in CommunityPageView
change assertListEqual to assertItemsEqual in unittest