feat: add optional django-import-export with generic FK-context import support

[gourav-sraoss]

Summary

Adds an opt-in django-import-export integration to generated projects, including a reusable contrib package that solves a common pain point: selecting one FK value once in the import wizard and having it automatically applied to every imported row — without requiring that column in every line of the CSV file.

Default behaviour is unchanged ("use_django_import_export": "n").

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Motivation

Admin-side CSV import/export is one of the most frequently needed features for operational Django projects — data migrations, bulk updates, support workflows. Yet wiring it up correctly (especially the shared-FK pattern) requires boilerplate that every team re-writes. This PR gives generated projects the integration out of the box, and eliminates that boilerplate entirely via a two-class contrib package.

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What changed (2 commits, 5 files changed, 185 insertions)

1 — Template option and app wiring

File	Change
cookiecutter.json	New option use_django_import_export (default n)
pyproject.toml	Conditionally adds django-import-export dependency
config/settings/base.py	Conditionally adds import_export to THIRD_PARTY_APPS

2 — Generic FK-context contrib package

When the option is enabled, the generated project gains a ready-to-use contrib package:

[browniebroke]

I used django-import-export in a few project, but I'm not convinced it should be added to the template.

While making it a project option could be a good compromise, a new option always adds complexity in terms of testing, and adds a question when people generate a new project.

At this stage, I don't think it's worth it, but I will leave this open for while to let people give feedback.