Change Skin -> Theme

To better align with commonly used terminology in industry renamed all references from Skin -> Theme.

Oqtane.Server/Repository/IThemeRepository.cs

@@ -0,0 +1,10 @@
+using System.Collections.Generic;
+using Oqtane.Models;
+
+namespace Oqtane.Repository
+{
+    public interface IThemeRepository
+    {
+        IEnumerable<Theme> GetThemes();
+    }
+}