chore(deps): replace dependencies that use vulnerable lodash.template

[barronhagerman]

Summary

This removes the use of lodash.template by replacing the following dependencies:

• Replaces gulp-cssmin with gulp-cleancss
  • The resulting CSS in dist/css/style.min.css is slightly different, but it seems to simply involve reorganization of the CSS
• Replaces gulp-header with gulp-prepend

Addresses https://github.com/Banno/jha-design/security/dependabot/79

How to test

1. Check out master and run a dev server from the dist/ directory, such as npx http-server -p 3000
2. Load the web page
3. Check out this branch, repeat steps 1 and 2, and compare the CSS