Added manifest and serviceworker for PWA support

[bostrot]

Changes

This adds the minimal requirements for PWA apps:

The serviceworker is the bare minimum and would not allow offline use yet.

• [ ] The manifest should also include icons and splashscreens at different sizes which aren't added yet.

Tests

• [X] This PR does not require tests

Changelog

• [ ] Entry has been added to changelog

Documentation

• [X] This change does not need a documentation update

Dark mode

• [ ] The UI has been tested both in dark and light mode