Previously the various settings were accessed in the places, where they
are used.  This also included the specification of default values.
Now all settings and their defaults are evaluated in a single location
(grouprise.core.settings).

Most users of grouprise settings should use the following pattern:

  from grouprise.core.settings import FOO

If test-related mocking of settings is relevant, then the module should
be imported instead (in order to allow dynamic attribute overrides):

  import grouprise.core.settings as grouprise_settings

See #648