Add GitHub Actions workflow for auto deployments

[jtpio]

Add a GitHub Actions workflow to automatically deploy the gallery when pushing to master.

This is a very simple fix #57, but good enough for now. We can build on top of that afterwards, or even move the deploy script to the voila-gallery.org-deploy repo.

This is what the workflow does:

• SSH into the existing instance
• Reinstall the gallery package
• Restart JupyterHub
• Test it's up again after 5 seconds

[jtpio]

@SylvainCorlay @maartenbreddels I need admin access on this repo to add the secrets:

[jtpio]

Example runs in this fork PR (used for testing, to be deleted later): jtpio#1

[SylvainCorlay]

@jtpio you now have admin priviledges to this repository.

[jtpio]

@SylvainCorlay thanks!

[jtpio]

Secrets have been added.

I think we can press the green button and see how it goes.

[jtpio]

As briefly mentioned in the first post of the issue, this is a bit of a "poor man's solution" to doing deployments (SSHing into a machine). But it's a start.

Also to give some context on why using GitHub actions:

• There was no CI for the gallery before (starting from scratch)
• Actions are easily accessible on GitHub, no extra application to install
• The SSH action (appleboy/ssh-action) is exactly what we need here

[jtpio]

The latest changes (Jupyter logo added last Friday) that were not deployed are now live: