Feature/new web UI

[simonmicro]

This one still in progress - but soon :tm: ...

Roadmap:

• [x] finish c++-code
• [x] finish angular-code
• [x] ui-repo: Write README
• [x] ui-repo: CI/CD?
• [x] ui-repo: releases? -> master/develop branch scheme
• [x] ui-repo: publish!
• [x] docs: add new screenshots
• [x] docs: some way to build locally? is that documented enough?
• [x] readme/scripts: make sure they are referenced / submodule pulls work properly
• [x] api/ui to reset/export/import settings?
• [x] ui: does the default value work correctly?
• [x] api/ui: reboot button
• [x] api/ui: version-number-contract (mismatched numbers -> warning in ui)
• [ ] ~add web-ui screenshots into main repo too~

Sneak-Peek into the new UI (running with dummy-data in that screenshot; I was too lazy to connect my watch :grinning: ):