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/EnvelopeHistory.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;
@@ -38,7 +39,7 @@ namespace EnvelopeGenerator.Domain.Entities
         public string Comment { get; set; }
 
         [ForeignKey("UserReference")]
-        public virtual EGUser Sender { get; set; }
+        public virtual User Sender { get; set; }
 
         [ForeignKey("UserReference")]
         public virtual Receiver Receiver { get; set; }