Refactor using directives and add User entity

- Updated `using` directives in `Config.cs` and `EnvelopeType.cs` to include additional namespaces and removed `DigitalData.Core.Abstractions`.
- Adjusted formatting for `StatusName` and `IsAlreadySent` properties in `Envelope.cs` for consistency.
- Simplified `User` property in `Envelope.cs` by removing the namespace prefix.
- Introduced a new `User` class in `User.cs` with various properties and data annotations for database mapping.
- Removed the `<Nullable>` property from `EnvelopeGenerator.Domain.csproj`, which may impact nullability handling.

EnvelopeGenerator.Domain/Entities/Envelope.cs

@@ -21,7 +21,7 @@ namespace EnvelopeGenerator.Domain.Entities
         public int Status { get; set; }
 
         [NotMapped]
-        public string StatusName  => ((Constants.EnvelopeStatus)Status).ToString();
+        public string StatusName => ((Constants.EnvelopeStatus)Status).ToString();
 
         [Required]
         [Column("ENVELOPE_UUID", TypeName = "nvarchar(36)")]
@@ -92,7 +92,7 @@ namespace EnvelopeGenerator.Domain.Entities
         /// The sender of envelope
         /// </summary>
         [ForeignKey("UserId")]
-        public DigitalData.UserManager.Domain.Entities.User User { get; set; }
+        public User User { get; set; }
 
         [ForeignKey("EnvelopeTypeId")]
         public EnvelopeType EnvelopeType { get; set; }
@@ -101,7 +101,7 @@ namespace EnvelopeGenerator.Domain.Entities
         public string EnvelopeTypeTitle => EnvelopeType.Title;
 
         [NotMapped]
-        public bool IsAlreadySent => Status > (int) Constants.EnvelopeStatus.EnvelopeSaved;
+        public bool IsAlreadySent => Status > (int)Constants.EnvelopeStatus.EnvelopeSaved;
 
         public IEnumerable<EnvelopeDocument> Documents { get; set; }