Notification Email Improvements

GitHub has enhanced its notifications system with several improvements, including a new email backend that supports rich formatting and smart mail headers. Email notifications for issues, pull requests, and comments now come with both HTML and plain text options, supporting Markdown elements and GitHub-specific features like emojis and @mentions. The updates also refine the use of mail headers, utilizing Bcc and Cc fields to better signal importance and enable advanced filtering in mail clients, with examples including Gmail's mute and priority features. Additionally, an embedded invisible image in HTML emails allows for automatic read status synchronization between email clients and GitHub's notifications interface, with image viewing permissions needed for this feature to function. These enhancements aim to improve user experience across various email platforms.