Add "APPNAME" headings in `.env`, and move the "migrate" logic into `appvars_create.sh`

[CLHatch]

Add headings for each app in the .env in the form of:

#
# APPNAME
#

Pull request

Purpose Describe the problem or feature in addition to a link to the issues.

Approach How does this change address the problem?

Open Questions and Pre-Merge TODOs Check all boxes as they are completed

• [ ] Use github checklists. When solved, check the box and explain the answer.

Learning Describe the research stage Links to blog posts, patterns, libraries or addons used to solve this problem

Requirements Check all boxes as they are completed

• [ ] These changes meet the standards for contributing.
• [ ] I have read the code of conduct.

[CLHatch]

Pretty sure those comments have all been addressed.