Modify models.py

[sson68x]

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Closes #1511

Description

This pull request improves the Notification model in the CiviWiki codebase by addressing several issues related to readability, maintainability, and consistency. The key improvements include:

1. Refactoring activity_CHOICES: Hardcoded strings have been replaced with an Enum, ensuring consistency and reducing the risk of errors when modifying or using activity types.
2. Correcting Typographical Errors: Fixed a typo in the activity_CHOICES value "response_to_yout_civi", correcting it to "response_to_your_civi".
3. Improving Field Definitions: Removed redundant attributes (blank=True and null=True) from the created and last_modified fields, as these were unnecessary for fields using auto_now_add and auto_now.
4. Enhancing Professionalism: Replaced unprofessional comments with meaningful documentation or removed them entirely.
5. Clarifying Relationships: Added better documentation to the ForeignKey fields to improve clarity about their relationships.

These changes make the codebase more maintainable, professional, and easier to work with, aligning with best practices in software engineering. The pull request also includes updated unit tests to ensure the correctness of the changes and compatibility with the existing system.

[pep8speaks]

Hello @sson68x! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file project/notification/models.py:

Line 10:1: E302 expected 2 blank lines, found 1

[brylie]

Thanks! I'm considering reviving this project. Would you be interested in contributing?