Deploy on GH pages

[Tanja-4732]

Objective

The application needs to be compatible with static serving; see #143

Host this project on GH pages:
This project would then be hosted at https://bernd-l.github.io/ExDateMan/.

Tasks

• Set up a gh-pages branch
• Make the branch the default option for GitHub Pages in the repo settings
• Use this GitHub Action to automate the build & deploy: https://github.com/marketplace/actions/angular-deploy-gh-pages-actions
• Have it run at least once
• Checkout the branch locally
• Create a symlink for the 404 page (run ln -s index.html 404.html)
• Commit the symlink
• Push to origin/gh-pages
• Add a link to the README (https://bernd-l.github.io/ExDateMan/)