Enable files tab for terminal allocations and tasks

[schmichael]

Nomad version

Nomad v1.0-dev

Issue

The UI displays an error when attempting to browse files for a terminal allocation or task:

Since the Nomad client may have GC'd any terminal allocation's files already, this API/UI can only be a best effort attempt and may still fail. However, retrieving files from terminal allocations is one of the only ways to debug configuration file rendering issues when using a template stanza. A best effort is better than nothing.

Reproduction steps

1. Start an example job
2. Stop it
3. Visit http://localhost:4646/ui/allocations/---/fs

[github-actions]

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