Dashboard rework

This PR reworks the dashboard project significantly

Affected files

This PR only touches on files in the ./dashboard directory, sensor and server projects remain unaffected

restores features on the dashboard (plenty of bugfixes)
replaces next.js with vanilla reactjs, built using vite, which allows static hosting again (such as through github pages)
adds login-page / user authentication
adds additional features (network selection, settings/selection-persistance, admin-page)
adds performance-improving features (selective data loading)
adds a create_user.sh script for manually adding users from the commandline
makes the API-server-URL configurable via env-vars (rather than hard-coded)
fixes issues with github-pages deployment