Reader Comments: Indent cells according to comment depth

[dvdchr]

Refs #17476

As titled, this PR implements cell indentation based on the Comment's depth property. The indentation width is adjusted to be 15.0 as per the design (instead of 40.0 previously).

To Test

• Enable the newCommentThread feature flag.
• Go to Reader, and go to any comment thread that contains nested comments.
• 🔍 Verify that the cells are correctly indented based on their depth.

Regression Notes

1. Potential unintended areas of impact
   n/a. This feature is hidden behind a flag.

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   n/a. This feature is hidden behind a flag.

3. What automated tests I added (or what prevented me from doing so)
   n/a. This feature is hidden behind a flag.

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

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You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[ScoutHarris]

Nice!