Add django-linear-migrations

Description

This task is to add django-linear-migrations to the webapp boilerplate, after it proved very useful in https://gitlab.developers.cam.ac.uk/uis/devops/digital-admissions/pools/smi/-/issues/714.

Further details

This also includes some documentation in the guidebook how to use this, and how to include this in an existing project.

Task list

• Add django-linear-migrations to boilerplate
• Add documentation in guidebook explaining how to use it and how to implement this in existing projects

Acceptance criteria

• django-linear-migrations is installed and has created a max_migration.txt file.
• Guidebook guidance available for migrating an existing project to use this

Links/references