DF-1051 - Add using statements to the LK media library view

Added @using Kentico.Content.Web.Mvc and @using Kentico.MediaLibrary statements to the view for better clarity in the documentation (not everyone will have these namespaces in their view web.config). Based on customer feedback.

Author: DavidBKentico

samples/LearningKit/Views/MediaLibrary/ShowMediaFiles.cshtml

@@ -4,8 +4,11 @@
 }
 
 @*DocSection:ShowMediaFiles*@
+@using Kentico.Content.Web.Mvc
 @using Kentico.MediaLibrary
+@using Kentico.Web.Mvc
 @using CMS.MediaLibrary
+
 @model IEnumerable<MediaFileInfo>
 
 <h2>Media library files listing</h2>