Add split_preview_preprocess_html()

Remove the toolbar if the 'split_preview' route parm is true.

Closes #3274079

css/live_preview.css

@@ -17,9 +17,6 @@
   right: 0;
   z-index: 1000;
   height: 100%;
-  display: -webkit-box;
-  display: -ms-flexbox;
-  display: flex;
   width: 100%;
 }
 
@@ -28,12 +25,8 @@
 }
 
 .live-preview__actions {
-  position: absolute;
-  top: 0;
-  right: 0;
   padding: 2px 0;
   background: black;
-  width: 100%;
   text-align: right;
   transition: all 0.5s;
 }