Check whether part has been marked as disabled on part detail page.

Only show update button, if part is not marked as disabled. If it's disabled, show also a warning on top of page.
Show n/A instead of None if there is no detail information available for a part.
Extend Part.get_fields() to don't return disabled field
Mark undefined unit of a part with n/A instead of ---
Fix tests of partsmanagement for dynamical PARENT_DELIMITER-option inside settings.py
Add some structure via comments to test-file

No code change. Just trying to help orientating inside file.
Fix a little typo on part detail page
Add a testcase for Part.get_fields()
Add testcase for storage recursivness