move scss about page preview button to molecules

src/client/styles/scss/_layout.scss

@@ -88,12 +88,6 @@ body {
   }
 }
 
-// page preview button in link form
-.btn-page-preview svg {
-  width: 18px;
-  height: 18px;
-}
-
 // printable style
 @media print {
   padding: 30px;

src/client/styles/scss/molecules/page-preview-btn.scss

@@ -0,0 +1,5 @@
+// page preview button
+.btn-page-preview svg {
+  width: 18px;
+  height: 18px;
+}