[WIP] Add "theme_experiment" addon support

[trickypr]

What is this This is an implementation with the goal of closing #143. You can find more information by reading it. tl;dr: There is a way of distributing themes as extensions to Firefox Nightly and Firefox forks.

Specific details This adds the ability for a user to select the browser they are using and be provided with install instructions for using "theme_experiment" addons within their browser. There is currently no way to have themes that only support "theme_experiment" but themes without "theme_experiment" are not impacted in any way.

Checklist

• [x] Themes can be added to themes.json
• [x] The tests are updated to ensure they work as intended (depends on xpi-creator)
• [x] "theme_experiment" addons have correct install instructions
• [ ] Method for filtering for only "theme_experiment"
• [ ] Instructions for theme developers to package
• [ ] Discussion around the UI / UX of this implementation

Live demo: https://trickypr.github.io/FirefoxCSS-Store.github.io/