Screenshots updated to Nova

[rebloor]

This change:

• updates all Extension Workshop screenshots that included visual components of the Firefox UI to use the Nova interface, except for theme-related updates, which are provided in Theme updates for Nova Theme updates for Nova #2452.
• updated the debugging page, which previously showed the toolbox opening in a tab; it now correctly shows it opening in a window.
• removed the embedded YouTube videos illustrating how to install and reload a temporary extension, which included the old Firefox UI. The motivation was that these videos provided little benefit over the text provided, given the effort of recreating them.
• remove the unlimited storage permission from the Choose the right permissions page, given that it's not notified (and doesn't have an effect in Firefox).