e2e testing

[bengaid]

• [x] stop endpoint for flask server (from stopping from Node.js)
• [x] Serve front-end build with Express
• [x] Mocha e2e test suite using selenium
  • [x] Open a set of dashboard and compare screenshot with reference
  • [x] Sample test using selenium to pilot the dashboard editor
• [x] Run tests using github actions
  • [x] Trigger on pull requests ready for review