Add progress bar and fetch button

[krassowski]

Closes #6

Adds dependency on git-python

The GIF demonstrates:

• progress bar on pull
• then I revert to a previous revision using jupyterlab-git
• refresh page
• the pull button is again active
• clicking it now uses the nbgitpuller logic to pull the latest version - the progress bar for this one is not very good yet, but it is generally much faster so should be less annoying

[github-actions]

👈 Launch a Binder on branch nebari-dev/jupyterlab-gallery/progress-and-buttons