Update layout of DeviceSettingsPage; add labels to sections; add tests

[jonboiser]

Summary

1. Updates layout of DeviceSettingsPage (nest dependent device setting, similar to facility setting #7247)
2. Adds labels for the "External devices" section and the "Default landing page" section
3. Adds tests that should cover state hydration after fetching device settings, as well as re-saving them (and does this by simulating a user clicking on the forms).

Screenshot:

Some videos:

1. Checking that the changes are persisted + hydrated correctly
2. Using the form with keyboard only

References

Fixes #7247

Reviewer guidance

Due to #8087, it might not be able to confirm the settings have changed by using the app. Instead, you might need to go to the API explorer (http://localhost:8000/api_explorer/) and call the devicesettings end point.

Video of me confirming this on Swagger API Explorer

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Testing checklist

• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included
• Critical user journeys are covered by Gherkin stories
• Critical and brittle code paths are covered by unit tests

PR process

• PR has the correct target branch and milestone
• PR has 'needs review' or 'work-in-progress' label
• If PR is ready for review, a reviewer has been added. (Don't use 'Assignees')
• If this is an important user-facing change, PR or related issue has a 'changelog' label
• If this includes an internal dependency change, a link to the diff is provided

Reviewer checklist

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md

[sairina]

LGTM!