Add "Seats in use" view to its own page

What does this MR do?

This is the implementation MR of #268245 (closed)

Screenshots

/billings (notice the "See usage" button)	/seat_usage

Does this MR meet the acceptance criteria?

Conformity

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