Improve create_migration script

It now:

- Restores the previous versions of Django and South
- Uses a bash array to reduce duplication

(Took the bait from 722c058e5ae1b5a3ac641ec84f42d2d972a5d483)
Start using TODO to flag event-pending changes

Motivated by 51bf7280, this change adds a new make target to find parts
of the codebase that need changing after some event has happened. It
also adds an example TODO.