Refactor user-related classes and properties

- Added parameterless and user-initializing constructors to `EGUser`.
- Removed unnecessary `using` directives and `#if NETFRAMEWORK` from `EGUser.cs`, `Envelope.cs`, and `EnvelopeHistory.cs`.
- Replaced `EGUser` property with `User` in `Envelope.cs` and `EnvelopeHistory.cs`.
- Retained default value for `CURRENT_WORK_APP` in `Envelope.cs`.

EnvelopeGenerator.Domain/Entities/Envelope.cs

@@ -1,4 +1,5 @@
-using System.ComponentModel.DataAnnotations;
+using DigitalData.UserManager.Domain.Entities;
+using System.ComponentModel.DataAnnotations;
 using System.ComponentModel.DataAnnotations.Schema;
 #if NETFRAMEWORK
 using System;
@@ -113,7 +114,7 @@ namespace EnvelopeGenerator.Domain.Entities
 
         // TODO: * Check the Form App and remove the default value
         [ForeignKey("UserId")]
-        public EGUser User { get; set; } = new EGUser();
+        public User User { get; set; }
 
         [ForeignKey("EnvelopeTypeId")]
         public EnvelopeType EnvelopeType { get; set; }