Added an "About" popup

CHANGES.md

prepare_release.py

src/digi_edit/static/app.js

src/digi_edit/static/app.js.map

src/digi_edit/static/changes.json

src/digi_edit/static/theme.css

src/digi_edit/templates/ui.jinja2

src/frontend/src/App.vue

src/frontend/src/store/index.ts

src/theme/_flex.scss

src/theme/_form.scss