Artifacts: Disable browse button if clicking it will lead to a 404

The following discussion from !101689 (merged) should be addressed:

• @anna_vovchenko started a discussion: (+1 comment)

  Note (non-blocking): is it possible to not return the path if it leads to 404? I think it's out of scope of the current MR though.

Technical proposal

Context:

The browse page uses the job's metadata.gz file as a data source - deleting this file is what makes the browse page 404.

Steps:

• map the presence of the metadata.gz file for a job to a boolean (the same way we already do the artifacts.zip archive file)
• disable the browse button for a job if it this boolean is false