Make Malware Verdicts auditable

[xmunoz]

What's the problem this feature will solve?
Because of ac9c649, Malware Verdicts are completely removed once the associated object is deleted (CASCADE on delete). This eliminates the ability of to perform audits of verdicts or evaluate the efficacy of checks.

Describe the solution you'd like
Once #4440 is addressed, the schema of malware_verdicts should be modified such that verdicts are never removed from the database.

[dstufft]

This is gone now #13647