Fix tab traps in JupyterLab

[isabela-pf]

Summary

Based on sprint planning discussions in #114.

Quoting from #114:

Fix tab traps in the Lumino menu bar and then in JupyterLab if more adjustments are needed (@gabalafou)

Acceptance Criteria

Tab trap-focused PRs are merged in relevant projects that build JupyterLab and we've tested to confirm the changes.

Tasks to complete

This would need to be ironed out if we choose to pursue this is a sprint.

[gabalafou]

Here are some sub-tasks that would need to be completed before I would consider this done:

• Fix the keyboard trap in an open notebook (there needs to be a simple way to get out of the notebook and to other parts of the UI using only the tab, arrow, or ESC keys. See WCAG 2.1.2 No Keyboard Trap (Tracking in Tab trap in JupyterLab notebook code cell #168)
• Fix the keyboard trap in the terminal. It's possible to close the terminal by typing CTRL+D, but this probably needs to be communicated, or we need to provide a more standard keyboard shortcut. (Tracked in Tab trap in JupyterLab terminal #169)
• More extensively audit the UI for other keyboard traps.
• Provide visual indicator of moving tab focus. Right now, it's difficult to know where you are in the UI as you tab through it. (Tracked in Focus not always visible on JupyterLab initial page #167 - PR and followed by more fixes in this PR)