Auto-takedown logic for reported notes

[ShriramJana]

Automatically hide notes after they receive N reports. Notes should be hidden, not deleted.

Once users can submit reports via #159, we need a mechanism to automatically suppress content that receives multiple reports.

• Add a hidden boolean column to the file table (default false)
• After a note reaches a threshold number of reports (e.g. 5), automatically set hidden = true
• Hidden notes should not appear in search results, course/professor pages, or any public listings
• Hidden notes should still be visible to admins in the moderation dashboard (Admin moderation dashboard #162)
• The uploader should be informed that their note was hidden
• Admins can manually unhide a note from the moderation dashboard if the reports were unwarranted

[TyHil]

Decided on 3 reports to auto-hide a note