Add staging configuration and deployment

[catarak]

Nature of issue?

• New feature request

New feature details:

As suggested in #1075, this is a task to set up feature/public-api as a staging branch.

• add staging_deploy script/task to Travis, which will be extremely similar to deploy.sh
• update Kubernetes cluster config with new deployment/namespace for staging
• add new repository on Dockerhub for staging image
• add these variables to Travis

[catarak]

The Docker Hub staging image is p5.js-web-editor-staging

[catarak]

okay, i think is good to go.