This commit introduces the following changes which allows a site
administrator to mark `Upload` records with the `s3_file_missing`
verification status which will result in the `Upload` record being ignored when
`Discourse.store.list_missing_uploads` is ran on a site where S3 uploads
are enabled and `SiteSetting.enable_s3_inventory` is set to `true`.
1. Introduce `s3_file_missing` to `Upload.verification_statuses`
2. Introduce `Upload.mark_invalid_s3_uploads_as_missing` which updates
`Upload#verification_status` of all `Upload` records from `invalid_etag` to `s3_file_missing`.
3. Introduce `rake uploads:mark_invalid_s3_uploads_as_missing` Rake task
which allows a site administrator to change `Upload` records with
`invalid_etag` verification status to the `s3_file_missing`
verificaton_status.
4. Update `S3Inventory` to ignore `Upload` records with the
`s3_file_missing` verification status.
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