Fix notebook cell divider size

[jonah-iden]

What it does

Previously the cell divider height was to small to contain the content so hovering over the last divider in a notebook resulted in a scrollbar being created which changed the width of the notebook editor widget.
This should be fixed now

How to test

1. open a notebook with more cells than fit on the screen.
2. Hover over the last cell divider at the bottom of the notebook
3. No extra scrollbar should appear, and the editor width should not change

Follow-ups

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[msujew]

LGTM 👍