rbiyovskiy/739 workshop control of the available places (#749)

* Model, DTO and mapping

* Changes in the services

* Tests' changes

* Migration

* Deleted migration

* Restore using OutOfSchool.Common.Enums

* Migration

* fixed test

Author: RBiyovskiy

OutOfSchool/OutOfSchool.DataAccess/Models/Workshop.cs

@@ -101,6 +101,8 @@ public class Workshop : IKeyedEntity<Guid>, IImageDependentEntity<Workshop>
 
     public WorkshopStatus Status { get; set; }
 
+    public uint AvailableSeats { get; set; } = uint.MaxValue;
+
     public virtual Provider Provider { get; set; }
 
     public virtual InstitutionHierarchy InstitutionHierarchy { get; set; }