Create Team and TeamMembership models

[AdrianDAlessandro]

Description

This PR creates the Team and Team Membership models required to enable Teams features.

Each Team can have any number of members and any number of those members can be a manager.

None of this is visible to users yet, other than admin users that can see these in the database.

Fixes #569

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

❌ Patch coverage is 96.66667% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
main/models/team_models.py	91.66%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!

[Sahil590]

From what we discussed earlier
LGTM!