JavaScript v3.8.0

We've finally switched from CoffeeScript and rebuilt the widget in pure JavaScript. It was tough, but we did it.
We preserved backward compatibility for the public API except for the undocumented plugin method that could not be moved as-is. If you are using the method and experience problems with the new widget version, feel free to open a new issue.

Changed

• Migrated from CoffeeScript to JavaScript. See #567.
• Switched to ES Modules instead of namespaces. See #569.
• Migrated templates to tagged HTML strings from EJS. See #603.

Fixed

• Improved widget accessibility. Big thanks to @erin-doyle. See #594, #595 and #611.
• Updated Spanish locale. Thanks to @HerrLindner. See #550.
• Updated German locale. Thanks to @thomasklemm. See #580.
• Fixed effects tab locales. See #613.