Website: switch to JS site

To allow easier maintenance, we'll be switching from static html site to one that is JS powered, taking application data directly from Flathub. Some background discussion here.

• Move and update the website-json branch to use actual flathub api rather than local placeholder json.
• Set up development environment CI to be able to test the flathub api integration. The production site circle.gnome.org has been set up, but we need to be able to test changes before deploying to the production site.