Transition to github actions, removing travis dependency

[joncamfield]

• switch default to source branch
• duplicate current gh-pages branch to backup branch
• enable read/write environment for gh-pages
• Follow https://gohugo.io/hosting-and-deployment/hosting-on-github/ to set up workflow
• see also the joncamfield fork; perhaps pull request that in once stabilized/cleaned
• Test with basic deploys
• test with content changing deployes
• remove backup branch
• remove depricated master branch