Add shared package for database models

[AndyRae]

Changes

• Moves the database models to a app/shared package
• Add the shared package to the webapp and workers dependencies, updating imports accordingly
• Add a database test function to test with a deployed Azure function

Closes #662
Tracked by #612

The idea will be to use shared for any shared code between Django / Azure functions.
Retaining the app/api/data package for now, as it's attached to migrations (a future problem)

So these changes are mostly changing imports, and the interesting bits are dependency changes.

Checks

Important: please complete these before merging.

• Run migrations, if any.
• Update changelog.md, including migration instructions if any.
• Run unit tests.

[github-actions]

Tests	Skipped	Failures	Errors	Time
25	1 💤	0 ❌	0 🔥	2.104s ⏱️