Refactor controllers and views for MediatR integration

Updated several C# controllers to use the new
`DigitalData.Core.Abstraction.Application.DTO` namespace
and removed references to `DigitalData.Core.DTO`. Added
`[Obsolete("Use MediatR")]` attributes to indicate a shift
towards MediatR for request handling. Improved error
handling and code organization in key methods. Updated
Razor view files to reflect namespace changes for
consistency across the application.

EnvelopeGenerator.Web/Views/TestView/DebugEnvelopes.cshtml

@@ -1,5 +1,6 @@
 @using EnvelopeGenerator.CommonServices;
-@using static EnvelopeGenerator.CommonServices.Constants;
+@using EnvelopeGenerator.Domain;
+@using static EnvelopeGenerator.Domain.Constants;
 @{
     ViewData["Title"] = "Debug";
 }