Add details table to seat usage table

What does this MR do?

Add a drilldown table for displaying details about a seat in use. This table show details about a particular member, memberships.

Closes #321560 (closed)

Screenshots (strongly suggested)

See it in action

seat_usage_details-2021-04-15_14.17.45

How to check it locally

• Ensure you have enabled the Check feature availability on namespace plan in the admin settings under the Account and limit section.
• Navigate to the Billings page of the desired namespace. NOTE: You must have running CustomersDot locally to access this page otherwise just edit the URL manually and change billing to seat_usage: http://localhost:3000/groups/group_with_ultimate_plan/-/billinng -> http://localhost:3000/groups/group_with_ultimate_plan/-/seat_usage

Now try to add some members to a namespace, subgroup or a project.

Does this MR meet the acceptance criteria?

Conformity

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• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
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