Add snippets_size to namespace usage statistics views

What does this MR do?

In this MR we add snippets_size to Namespace usage statistics views.

This is how the UI looks with the new color:

Refs #224656 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content