[C2] Make attempt_file and attempt_feedback entities into resources

[ywarnier]

We noted a bit late that attempt_file and attempt_feedback are data (files) that could contain personal information as they represent answers, by users, to test questions.

This is (not very intuitively) a problem in terms of personal data protection and it must be mitigated quickly, even at the cost of introducing more database changes in a Beta version.

What to do:

• declare new resource types for attempt_file and attempt_feedback
• update attempt_feedback to add resource_node_id
• migrate assets (from the assets table) that match attempt_feedback to the resource_node/link/file tables and update attempt_feedback.resource_node_id, including moving (and transforming the names) files from var/upload/assets (I suppose that's where they are stored) to var/upload/resource
• update attempt_feedback to remove asset_id
• update attempt_file to add resource_node_id
• migrate assets (from the assets table) that match attempt_file to the resource_node/link/file tables and update attempt_file.resource_node_id, including moving (and transforming the names) files from var/upload/assets (I suppose that's where they are stored) to var/upload/resource
• update attempt_file to remove asset_id
• update the Exercise code to take into account these attempt_file and attempt_feedback are now resources

[christianbeeznest]

It is checked with this PR #7124 , requires more testing in migration.

Thanks for confirmation.