Standardize on mechanism for specifying generic related objects during CSV import

[jeremystretch]

Proposed Changes

When importing objects in CSV format, require specifying the related object type in the form <app>.<model> for consistency. The following forms need to be updated:

• CableCSVForm: Import A/B terminations using model name only
• IPAddressCSVForm: assigned_object_type does not use CSVModelChoiceField
• SecretCSVForm: Object type uses model name only

Justification

Ensures a consistent UX for importing all types of objects.