feat(billing): update frontend for polar migration

Update billing types, remove invoice preview, replace Stripe invoice
display with Polar orders, update tax ID from array to single object,
remove upcoming invoice amount display.

lib/api/admin.ts

@@ -2,8 +2,8 @@ import { authFetch } from './client'
 
 export interface AdminOrgSummary {
   organization_id: string
-  stripe_customer_id: string
-  stripe_subscription_id: string
+  billing_customer_id: string
+  billing_subscription_id: string
   plan_id: string
   billing_interval: string
   pageview_limit: number