Rename Cultures class to MultiCulture throughout codebase

Refactored all references from Cultures to MultiCulture in controllers and base classes. Updated the class definition in MultiCulture.cs to improve clarity and consistency in culture-related functionality. No functional changes were made.

EnvelopeGenerator.Web/Models/MultiCulture.cs

@@ -0,0 +1,15 @@
+namespace EnvelopeGenerator.Web.Models
+{
+    public class MultiCulture : List<Culture>
+    {
+        public IEnumerable<string> Languages => this.Select(c => c.Language);
+
+        public IEnumerable<string> FIClasses => this.Select(c => c.FIClass);
+
+        public Culture Default => this.First();
+    
+        public Culture GetOrDefault(string? language) => this[language] ?? Default;
+
+        public Culture? this[string? language] => language is null ? null : this.FirstOrDefault(c => string.Equals(c.Language, language, StringComparison.OrdinalIgnoreCase));
+    }
+}