Replace User class with EGUser across the codebase

This commit introduces the new `EGUser` class, which replaces the existing `User` class in multiple files, including `EnvelopeModel.vb`, `State.vb`, and `UserModel.vb`. The `EGUser` class extends `User`, adding new properties while maintaining existing functionality.

Key changes include:
- Updated property types from `User` to `EGUser` in relevant models.
- Method signatures in `UserModel.vb` modified to accept and return `EGUser`.
- Domain entities in `Envelope.cs` and `EnvelopeHistory.cs` now reference `EGUser`.
- Complete removal of the `User.cs` file.
- Added a new package reference for `UserManager.Domain` in the project file.
- Updated `MYUSER` variable in `ModuleSettings.vb` to use `EGUser`.

These changes enhance the user model to better meet application requirements.

EnvelopeGenerator.Domain/Entities/EnvelopeHistory.cs

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