more buildings

Created all necessary buildings for a moon. The shared buildings have been subclassed, to make them assignable to a moon.
Fixed a bug in Home-view

``get_list_or_404()`` was used, but this makes no sense, if the user has no accounts... We must show the standard list (even if it is empty) and provide the possibility to create a new account.
Yeah, actually show that account creation link
Merge branch 'hotfix01' into feat_moons
little import fix
Created an AstronomicalObject as base for planets and moons

With this AstronomicalObject, no special classes for buildings are required, so they were removed
Apply the AstronomicalObject class