Remove unused "Level 4 file viewer" code

[evansd]

Previously, we had a system for viewing unreleased output files which involved users logging in to Job Server from within Level 4 (using a single-use token) and then using a "hybrid" JavaScript app which combined a remote UI with file data from a local API. This allowed viewing unreleased files without that data every leaving the secure environment.

Airlock replaces this wholesale by running the entire application within the secure environment.

This PR updated the UI to replace references to the old system with references to Airlock:

• Point to Airlock for viewing log outputs #4585

However it left in place of lot of now redundant code which should be cleaned up. A few examples are:

• The Workspace.get_files_url method and any tests which reference it.
• The backend_files.html template and whatever view loads it, and whatever URL config points to that view.
• The single-use token login UI.
• The signing.py utility code used by the WorkspaceBackendFiles view (see Review maintenance of signing.py #4602 for discussion)

Note that the single-use token API endpoint needs to remain as this is used by Airlock. As therefore does the single-use token generator UI. It's just logging in to Job Server itself using the token which is no longer supported.

Note also the the file viewer SPA needs to remain for now as this is used for viewing released output files. Longer term there's an ambition to replace this with something simpler.

Related Slack thread:
https://bennettoxford.slack.com/archives/C069YDR4NCA/p1725463761914569

[iaindillingham]

Thanks, @evansd. I think this is an issue for 🦖, so I've added it to the board. If you think differently, then let me know!

[evansd]

Ah, thanks @iaindillingham. You're absolutely right that this is natural 🦖 territory. I've just added a link to the associated Slack thread where we discussed this. Because I started pulling on this particular thread, and because it's motivated by Airlock, it's possible I'll take it on. Or some of it at least.

[inglesp]

Will any documentation changes be required?

[lucyb]

Will any documentation changes be required?

I expect any user-facing documentation will have been updated as part of the Airlock work. Although, that said, this page in the team manual should be updated, but as part of the Airlock work rather than this ticket.

Otherwise, I've had a look and I can't see any documentation that should be updated as part of this ticket.

[lucyb]

This ticket is likely to be tackled as part of this work, either way, it would be good for it to be updated as part of completing this ticket.