Includes the associated Go functions for inserting and selecting given a
specific user ID or subscription ID. Also adds required HTML tags and
HTTP queries for subscription process during user registration.
Uses gift package for server side resizing of selected letterhead
file. It should fit an image file with aspect ratio of 2:1 and be a
reasonable size. A 400x200 resizeToFit is used for now.
gift just uses the native image/draw package internally.
https://pkg.go.dev/image/draw