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
Apply AstronomicalObject (again...)
Use that astro_object
fixed planet overview
fixed planet buildings and planet defense
Make updates work again
Make empire view work again
Fixed planet deletion

Planet deletion has been working correctly, but you were able to delete all planets of an account, which renders the account invalid.

This is fixed by redirecting the user to account deletion confirmation. Even better would be to provide the user with the possibility to create a new planet on this page.
Creation of Moon
Make moon creation work
Create view stubs
Prevent more than one moon per planet
Moon settings and deletion of moons are working

The coords of Moons are not editable. But they should automatically update, if the coords of the planet change.

Signal or just modify the update-function?
Created Moon submenu
Apply moon menu to all planet views
Fetch planet list in moon views
Remove planet creation link from planet settings

Is already included in planet menu