Fix: unable to delete objects with comments

[psrok1]

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What is the current behaviour?

We're unable to delete object with comments.

What is the new behaviour?

Problem was caused by two relationships between Object and Comment table that were trying to reconcilate the fact that Object is deleted:

• comments with cascade="delete" included that deletes all comments that are orphaned
• comment_authors with default cascade setting that expects comments to be not deleted because they're treated like secondary table representing many-to-many relationship between objects and users.

These actions are in conflict that can be solved using passive_deletes=True, because foreign key constraints have already set ondelete behavior.

In addition, I set passive_deletes on comments as well to make sqlalchemy not even bother about that relationship (ondelete does the job passively from the SQLAlchemy perspective)

I don't know why it never happened before, maybe bug was somehow exposed by flattening the object inheritance model in 2.8.0.

Test plan

Closing issues

closes #697