Billing updates

- Break monthly and annual plans into two.
- Add upgrade and adjust additional users

src/Core/Models/Business/OrganizationChangePlan.cs

@@ -0,0 +1,13 @@
+using Bit.Core.Enums;
+using System;
+
+namespace Bit.Core.Models.Business
+{
+    public class OrganizationChangePlan
+    {
+        public Guid OrganizationId { get; set; }
+        public PlanType PlanType { get; set; }
+        public short AdditionalUsers { get; set; }
+        public bool Monthly { get; set; }
+    }
+}