Soft-delete and copy-on-write for comments/posts

[cysjonathan]

Issue

Students may sometimes delete/update comments/forum posts content. We sometimes need to know what the student originally wrote, and want to keep a history of the comments/posts.

Solution/Fix

1. When a user makes an update, we execute a copy-on-write to preserve the old content.
2. When a user deletes, we do soft delete instead of hard delete.

Considerations

Ensure that copy-on-write does not affect codaveri automated feedback generation/confirmation adversely. Existing behaviour should remain the same.
When copy-on-write is implemented for forum posts, to ensure that Forum-answer questions still works as expected when forum posts are copied-on-write.
Behaviour also needs to be probably specified and tested if the entire forum thread is deleted.