Set up multiple settings files

[MaxWasUnavailable]

Description

Please include a summary of the change and which issue is resolved. List any dependencies that are required for this change.

We can now switch between development & production settings through the DJANGO_SETTINGS_MODULE variable in docker-compose.yml.

The default is currently set to development settings.

Type of change

Please delete options that are not relevant.

• [x] New feature (non-breaking change which adds functionality)
• [x] This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce.

• Tested production settings, which caused an exception as expected (no secret key configured)
• Tested development settings, which behaved the same as prior to this change as expected
• Remade backend container

Checklist:

Please check [x] all that apply.

• [x] My code follows similar styling to existing code in this project
• [x] I have performed a self-review of my own code
• [ ] I have commented my code, at the very least in hard-to-understand areas
• [ ] I have made corresponding changes to documentation / READMEs
• [x] My changes generate no new warnings
• [x] I have remade my frontend and backend Docker containers to ensure my changes do not break the setup for these
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] New and existing unit tests pass locally with my changes
• [x] I have checked my code and corrected any misspellings