Add space group as another layer as unions of spaces

[cwang]

Description

Introducing *space group as a persisted union of spaces, used internally and externally as a way to remove direct dependency on a single space.

Works similarly to how *user group works in admin section.

A part of the solution for #46

Also as a temporary fix for #65

(old screenshot before renaming showing the UI)

(new screenshot showing the latest naming choices)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactor and code improvement (non-breaking change which improves code quality and/or performance)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation
• Tests
• Other chores such as maintenance

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[janaka]

Why call this "Engagement" and not "Space Union" or "Space Group"? for me personally it's not an intuitive name.

[cwang]

Why call this "Engagement" and not "Space Union" or "Space Group"? for me personally it's not an intuitive name.

Renamed to user groups and space groups from implementations to UI and anywhere in between.

We may change them on UI with more marketing-friendly names in the future, but I agree that sticking to what they really are when naming them in implementations is the best way forward.