feat: set jupyter notebook file browser root to /.

[ioga]

Description

Set the root of the jupyter notebook file browser to /, and the default starting location to the work_dir.
Pros:

• full file tree becomes visible
• default starting directory in the file browser, launched notebooks or terminals stays the same (work_dir), unless you navigate away in the browser
• you can always return back using the home icon, so it’s really hard to get lost
• if you navigate somewhere you don’t have write permissions, creating new notebook will return a “permission denied” error. there’s no new permissions obtained by the user (as they could always reach whatever location outside the work_dir using the terminal).

Background: https://hpe-aiatscale.slack.com/archives/C02T7L0593K/p1704929788275849

Test Plan

• Launch a notebook with or without work_dir config setting. Check if the default directories are still /run/determined/workdir, but you can access any directory you want (and have fs permissions to) from the file browser.

Commentary (optional)

Checklist

• Changes have been manually QA'd
• User-facing API changes need the "User-facing API Change" label.
• Release notes should be added as a separate file under docs/release-notes/.
  See Release Note for details.
• Licenses should be included for new code which was copied and/or modified from any external code.

Ticket

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (38e6825) 51.46% compared to head (6cf9cb5) 51.46%.
Report is 2 commits behind head on main.

Additional details and impacted files

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  Hits        80352    80352           
  Misses      74282    74282           
  Partials     1487     1487           

Flag	Coverage Δ
backend	36.14% <ø> (ø)
harness	64.25% <ø> (ø)
web	53.76% <ø> (ø)

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