Added description to allowed preview params

refs https://github.com/TryGhost/Team/issues/1101

- description field in Admin is being moved to an area which has a live preview so we want to be able to pass this through as a preview param
- uses `d` rather than `description` to follow the shorter param names pattern

core/frontend/services/theme-engine/preview.js

@@ -21,7 +21,8 @@ function getPreviewData(previewHeader, customThemeSettingKeys = []) {
         icon: 'icon',
         logo: 'logo',
         cover: 'cover_image',
-        custom: 'custom'
+        custom: 'custom',
+        d: 'description'
     };
 
     let opts = new URLSearchParams(previewHeader);