Add context menu link to 'Open with LÖVELINESS'.

* Also share more code between mimetype handler, drag+drop loader, and
  context menu loader.
* Fix some issues with the code injection
* Inject much less code by default, inject more after we know we are
  interested in modifying the page.
* Add key to manifest.json so it will have a consistent extension id,
  even when testing.
Compress .zip file more, and remove it before creating