589: add privacy policy and imprint links

[sascha11110]

Both URLs are configurable via LUNES_CMS_PRIVACY_POLICY_URL and LUNES_CMS_IMPRINT_URL. They are rendered in the admin user menu, the admin footer, the login page and on the public upload page.

Fixes: #589

[sascha11110]

TODOs:

• Privacy policy for CMS
• Configure on test and production environment

[deliverino]

LLM Review (verdigado-think)

The pull request follows the project's engineering standards and conventions.

Review Details:

• Django correctness: No model changes were made, so no migrations are required.
• Internationalization: The new strings "Privacy policy" and "Imprint" have been correctly added to lunes_cms/locale/de/LC_MESSAGES/django.po without fuzzy markers.
• Type safety: New functions in lunes_cms/core/context_processors.py and lunes_cms/core/utils.py are fully type-annotated.
• Code quality: The use of # pylint: disable=unused-argument in the context processor is justified as the request argument is required by the Django context processor signature.
• Security: No security-sensitive code (uploads, CSRF, keys) was modified.
• Testing: The new tests/core/test_legal_links.py provides good coverage for both the template rendering (authenticated and anonymous) and the helper logic.
• Commit message: The commit message follows the required convention (<ticket>: <summary> in lowercase).
• Labels: The changes are user-facing (adding visible links to the admin and public pages), so the absence of a label is correct as it belongs in the "Features, Fixes & Enhancements" category.

[LeandraH]

Looks great, thanks, even with the draft links! Also, great idea to move the links themselves to Salt!

One small nit, I think the links should probably also be shown on the log out page :)