Create the moderation decision model

[dhruvkb]

Fixes

Fixes #3636 by @sarayourfriend

Description

This PR

• adds the moderation decision models (abstract and concrete for both media types)
• adds a decision field on the report models
• deprecates the status field on the report models to use the decision model
  • usages of this field are replaced with the properties based on the decision field
  • this field can be removed in the future by the PR for Drop status column from content report tables #3642
• updates the admin UI for reports to account for changed fields
• refactors code to reduce duplication by moving common code to parent class
• fixes a validation issue in the AbstractMediaReport model (and update tests)

Note

The model uses some different names to match the naming scheme used by other API models. These are small, positive changes and should not block merge.

Testing Instructions

0. Check out the PR and run the dev server.
1. Observe the admin views for the reports and decisions.

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the default branch of the repository (main) or a parent feature branch.
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added or updated tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no visible errors.
• I ran the DAG documentation generator (if applicable).

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[sarayourfriend]

Looks pretty good so far, but I have two significant requests for changes:

1. Lets use full-names for the decision reason slugs, unless there's a really good reason not to that I am not seeing. If that is the case, then please document the reason in code.
2. We need to stick to standard Django tools for URL building, and should use request.build_absolute_uri for consistent and correct behaviour for each requester.

[AetherUnbound]

The model and associated changes all look good! I was able to run the app successfully locally.

[sarayourfriend]

LGTM! Just one suggestion to leave out the is_moderated property until we need it (or maybe even just in preference of using not is_pending as a clearer and "only one obvious way").

[github-actions]

This PR has migrations. Please rebase it before merging to ensure that conflicting migrations are not introduced.