fix styling issues in preview mode

[RyanCoulsonCA]

Related Item(s)

https://github.com/ramp4-pcar4/storylines-editor/issues/346, https://github.com/ramp4-pcar4/storylines-editor/issues/337, https://github.com/ramp4-pcar4/storylines-editor/issues/328

Changes

• The Return to top button will now be vertically aligned.
• Removed an unnecessary left margin from the active chapter button to ensure the text is centered in the button.
• Slides with no name will now be displayed in the table of contents as Untitled Slide.

Testing

These issues only exist in preview mode, so you won't be able to test this using the Storylines demo page. We can verify this is fixed during the next editor testing party.

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This change is 

[github-actions[bot]]

Your demo site is ready! 🚀 Visit it here: https://ramp4-pcar4.github.io/story-ramp/fix-346/#/en/00000000-0000-0000-0000-000000000000