[Tracking] C#/CDN: resolve TODOs

Generated at commit `57827bc9`:
- [ ] ``extra/admin-api/Spacebar.Cdn/Controllers/GetImageController.cs:49: // TODO: is message_id required? Can't tell from discord.food: /attachments/{channel_id}/[{message_id]/attachment_id}/{attachment_filename}``
- [ ] ``extra/admin-api/Spacebar.Cdn/Controllers/GetImageController.cs:53: // TODO: url signing, file type checks``
