Long-term goal: Browser extension to replace Java plugin

[ExE-Boss]

Depends on bug 1457500

This would solve the issue with Java 9+ no longer shipping with a plugin, and allow old websites which rely on the Java plugin to function properly and don’t have DoppioJVM.

This would serve a similar function to Mozilla’s Shumway (Mozilla's JavaScript Flash replacement, which also ships as a Firefox extension).

The extension would be implemented using the WebExtensions API.