Remove double import.
Add use_for_related_fields.

Use for related_fields lookups, this way foreignkeys can't be accessed
when the user doesn't have permission to view them.

Removed code duplication for geo manager.
Add information about `use_for_related_fields`.
Add some documentation for use_for_related_fields, and tests
Say 'use_for_related_fields' instead of 'this'
Double backticks
Merge pull request #3 from lizardsystem/deltaportaal

Make sure lizard-security also checks objects reached via a foreign key