make_fake_fixtures.py does not create accurate foreign key mapping for member chapters.

[Anna]

Summary of the Bug

The fake fixtures script does not create valid data - the members objects have chapter IDs that do not correspond to the actual chapter IDs.

See [this comment[(https://github.com//pull/129#issuecomment-706909833) for more context.

Steps to Reproduce the Behaviour

1. docker-compose build
2. docker-compose up
3. docker-compose exec db /bin/bash
4. mysql -p
5. use agagd; select chapter_id from members limit 5; select member_id from chapters;

The Expected Behaviour

chapter_id in the members table should correspond to a member_id in the chapters table.

[Anna]

#132 fixed this.