Admin Invoice Show Page: Update Invoice Status

[redferret]

As an admin
When I visit an admin invoice show page
I see the invoice status is a select field
And I see that the invoice's current status is selected
When I click this select field,
Then I can select a new status for the Invoice,
And next to the select field I see a button to "Update Invoice Status"
When I click this button
I am taken back to the admin invoice show page
And I see that my Invoice's status has now been updated