slightly more user-friendly mass mailing script

Prompted Gemini via aistudio.google.com to make the mass-mailer using Google Apps Scripts a bit more user-friendly - displaying a checklist of actions to be performed for sending the emails, and picking up the rich-text html from a draft email instead of needing to write out the html by hand. Creating a custom menu and displaying a sidebar UI are done by these functions, using this html.

The Draft email with subject MassMailTemplate must not be deleted, it must remain available throughout the run of the script. The body of the email is taken from this Draft.

Inline images inside the draft will be sent as attachments, and it is better to avoid attachments to prevent our mails from being marked as spam. Formatted text is fine.

The Checklist in the Mass Mailer menu item is just for display, unchecking all the items can be done manually before the next send.