We have been using YouTube's 'hqdefault.jpg' image which works consistently to provide a 480x360 thumbnail. YouTube does provide larger thumbnails, but not consistently for every video. By using og:image, we will fetch the best resolution YouTube can provide for each video.
This commit also refactors lazy-yt to re-use the thumbnail already existing in the cooked content. This means we get lazy-loading for free, and avoid hotlinking images to YouTube (when download remote images is enabled on the site).
The thumbnails were being correctly downloaded, but the URLs were not being substituted correctly in post_process_cooked because the lazy-yt output did not include the `onebox` class.
This will make a few minor improvements to the second factor user interface. Highlights include:
- Using the site's title to prefix the backup code filename. If non-ascii characters are detected, then prefix "discourse" instead.
- Add icons and change the text on some of the buttons for better clarity and consistency
- Add an education link to the security key modal
* DEV: Provide radix 10 argument to parseInt
* DEV: Provide radix 16 argument to parseInt
* DEV: Remove unnecessary parseInt calls
* Fix year formatting
parseInt was used here to convert decimals to ints
