Add related names to django models

[johannaengland]

In Argus most (all?) foreign keys and many to many fields have a related name in the form of <relation_as_lower_case_with_underscores>s set instead of the automatically generated <relation_as_lower_case>_set, which makes backwards calling of relations clearer.

Add this also to the nav models and amend existing calls to the new related names.

[johannaengland]

All related pull requests (#2539, #2540, #2541, #2542, #2544, #2546, #2547, #2549, #2550, #2551) are now merged, so this is done.