Add namespace id to tier_badge assignment event

What does this MR do and why?

Resolves #407831 (closed) and some refactoring.

Add namespace to assignment events for tier_badge experiment, and also ensure the assignment events are referencing the same namespace as the other tier_badge tracking events.

Namespaces referenced in other events (all frontend events) are autopopulated:

• On a group's overview page, the group is used as the namespace.
• On a project's overview page, project.namespace is used as the namespace.

Screenshots or screen recordings

• Sample assignment event with namespace id collected

How to set up and validate locally

1. Create a free group
2. Visit the overview page of that group
3. Check assignment event in Snowplow Micro (assignment events are backend events, not observable from browser extension)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.