Add GitHub Action to push to GESIS

[rgaiacs]

This is related to #2797.

The final workflow when #2797 is complete will be

1. PR in https://github.com/jupyterhub/mybinder.org-deploy is merged into main branch
2. GitHub Actions starts continuous deployment of https://mybinder.org
3. GitHub Actions push merged PR to https://git.gesis.org/methods-hub/interactive-environment
4. GESIS GitLab CI/CD starts continuous deployment of https://notebooks.gesis.org/

This PR requires a new secret named GESIS_METHODS_HUB_TOKEN that I'm sending in private to

• @manics
• @minrk

The secret was encrypted using the public SSH RSA key from GitHub:

ssh-keygen -f username -e -m pem > username.pem

cat GESIS_METHODS_HUB_TOKEN | openssl pkeyutl -encrypt -pubin -inkey username.pem > GESIS_METHODS_HUB_TOKEN.username.txt

To decrypt

cat GESIS_METHODS_HUB_TOKEN.username.txt | openssl pkeyutl -decrypt -inkey path/to/id_rsa > GESIS_METHODS_HUB_TOKEN.txt