fix context for multiple notebooks

[jonah-iden]

What it does

• Focuses and puts the first cell in edit mode when creating a new empty notebook
• fixes context for multiple open notebooks (this seemed to be broken before)

How to test

Create a new notebook. See first cell is selected and the editor is focused.

Create a second new notebook. cell toolbar should show the correct entries

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]

Looks good to me 👍