Show a summarize of Seats for Self-Managed and GitLab.com subscriptions
Problem
The initial designs in #679 (closed) show a summation of seats for subscriptions.
However this data isn't currently available from the backend: #1126 (closed) and making these calculations in the UI is error-prone; the worst thing we can do is show incorrect data.
Proposal
Once #1126 (closed) has been implemented, add Seats data to SubscriptionDetails.vue