Refactor EnvelopeHistory and update DbContext for migrations

Removed Sender and Receiver properties from EnvelopeHistory.
Added ActionDate and Comment properties. Introduced IsMigration
property in EGDbContext to conditionally configure foreign key
relationships for EnvelopeHistory. Updated EGDbContextFactory
to set IsMigration during context creation for migration operations.

EnvelopeGenerator.Domain/Entities/EnvelopeHistory.cs

@@ -38,10 +38,8 @@ namespace EnvelopeGenerator.Domain.Entities
         [Column("COMMENT", TypeName = "nvarchar(max)")]
         public string Comment { get; set; }
 
-        [ForeignKey("UserReference")]
         public virtual User Sender { get; set; }
 
-        [ForeignKey("UserReference")]
         public virtual Receiver Receiver { get; set; }
 
         [NotMapped]