feat: soft delete

[chintanjoshi-apphelix-2u]

Description

Implements soft delete functionality for discussion threads, responses, and comments using the is_deleted flag instead of permanently deleting records.
This enables safe deletion and restoration of discussion content while preserving existing data.

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Changes Made

• Added soft delete and restore functionality for threads, responses, and comments.
• Introduced support to view deleted content in the Learner tab using a new Active / Deleted filter.
• Implemented restore actions for deleted threads, responses, and comments.
• Added deleted count tracking and support for sorting by deleted count in the Learner tab.
• Implemented bulk delete and bulk restore actions for discussion content.
• Ensured nested responses and comments are handled correctly during delete and restore operations.
• Added API support to fetch deleted content for moderation workflows.

Note:
Features listed above (except soft delete itself) are available only to Staff, Admins, Moderators, and TAs.

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JIRA Tickets

• COSMO2-742 – Link
• COSMO2-769 – Link
• COSMO2-783 – Link

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Related Pull Requests

• feat: soft delete edx-platform#37
• feat: added soft delete functionality frontend-app-discussions#6

Merge checklist:
Check off if complete or not applicable:

• Version bumped
• Documentation updated (not only docstrings)
• Fixup commits are squashed away
• Unit tests added/updated