Admin Merchants: Top 5 Merchants by Revenue

[BrianZanti]

As an admin,
When I visit the admin merchants index
Then I see the names of the top 5 merchants by total revenue generated
And I see that each merchant name links to the admin merchant show page for that merchant
And I see the total revenue generated next to each merchant name

Notes on Revenue Calculation:

• Only invoices with at least one successful transaction should count towards revenue
• Revenue for an invoice should be calculated as the sum of the revenue of all invoice items
• Revenue for an invoice item should be calculated as the invoice item unit price multiplied by the quantity (do not use the item unit price)