Removed a call private set_installed_apps and the whole manual
pre-discovery at the cost of making the tests importable without
the Mezzanine setup.

The "import defaults" statements in tests are needed when testing
non-installed apps -- the test runner will attempt an import during
test discovery, before a list of what is to be tested is known.

Other than the just removed "pre-discovery", it would be possible
to solve this overriding unittest.TestLoaded (but a lot needs to
be overrided) or using sys.path_hooks (but this seems like an
overshot for such a case).
Rediscover page_processors after installing new apps during testing.

Also partly adapted defaults and page_processors importing to the
apps registry (1.7+ settings.INSTALLED_APPS should be replaced with
apps.get_app_configs(), apps.is_installed(), etc.).
Improved default in_menus test.

Abstracted the models to prevent them from poluting Site with
inexistant related object sets and made the class names more
descriptive.

Some improvements to MenusField to allow changes to PAGE_MENU_TEMPATES
and PAGE_MENU_TEMPLATES_DEFAULT past initialization would also make
them less tricky to test.