Include terms and conditions agreement checkbox in registration

[AdrianDAlessandro]

Description

This PR adds a checkbox to the registration form that must be checked for users to register. This checkbox is to agree to the pre-existing privacy policy and terms and conditions.

It also saves this with a corresponding timestamp in the database along with the user. So we know when they registered and when they checked the acceptance box.

Fixes #669

Type of change

• Documentation (non-breaking change that adds or improves the documentation)
• New feature (non-breaking change which adds functionality)
• Optimization (non-breaking, back-end change that speeds up the code)
• Technical work (non-breaking, change which is work as part of a new feature)
• Bug fix (non-breaking change which fixes an issue)
• Breaking change (whatever its nature)

Key checklist

• All tests pass (eg. python -m pytest)
• The documentation builds and looks OK (eg. mkdocs serve)
• Pre-commit hooks run successfully (eg. pre-commit run --all-files)

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added or an issue has been opened to tackle that in the future. (Indicate issue here: # (issue))

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[davehorsfall]

This is all working for me. I've just left some minor accessibility comments to address. Happy to approve after that.

[davehorsfall]

Consider adding following to both tags to improve accessibility:
target="_blank" rel="noopener noreferrer"

[AdrianDAlessandro]

Done!

[davehorsfall]

Thanks