Update Culture model and DI for non-nullable properties

Refactored Culture model to use required and non-nullable properties.
Removed nullable checks for Info in ShowEnvelope.cshtml.
Changed DI from Cultures to MultiCulture in _ViewImports.cshtml.

EnvelopeGenerator.Web/Views/_ViewImports.cshtml

@@ -7,6 +7,6 @@
 @using Microsoft.Extensions.Options
 @inject IStringLocalizer<Resource> _localizer
 @inject Microsoft.AspNetCore.Http.IHttpContextAccessor _accessor
-@inject Cultures _cultures
+@inject MultiCulture _cultures
 @inject IOptions<CustomImages> _cImgOpt
 @addTagHelper *, Microsoft.AspNetCore.Mvc.TagHelpers