Resolve "merge migrations so tests are running faster"

Closes #479 (closed)

This MR introduce "squashed" migrations for everything that happened before version 1.0.0.

This allowed to reduce test creation time from 18s to 4s.

The new migration was introduced semiautomatically (the code produced by ./manage.py did not work) and was optimized by merging some operations that were not picked by django.

The pipeline fails because on the latest python numpy cannot compile anymore. Not sure if we don't need to update numpy to fix this issue.