Display list of open-source dependencies and their licenses

[gavv]

Add "Acknowledgements" button to About page that shows screen like this:

(When you click on dependency name, you should see its license text)

We already have a JSON file with all necessary information (generated during build). We just need to implement UI.

How to:

• Add metadata/dependencies.json to flutter assets. See here: https://docs.flutter.dev/ui/assets/assets-and-images
• Add the button to about page. When it's clicked, load JSON from assets and build UI similar to the one on screenshot.