[Frontend] Styling Export button

fixes #973
- In progress:
- Added new download symbol to symbols font;
- Added export symbol to export button;
- Layout/markup/SASS for historical tabular
view modified;

platform/commonUI/general/res/sass/controls/_buttons.scss

@@ -109,6 +109,14 @@ $pad: $interiorMargin * $baseRatio;
             content: "\000039";
         }
     }
+
+    &.t-export {
+        &:before {
+            @extend .ui-symbol;
+            @extend .icon;
+            content:'\e623';
+        }
+    }
 }
 
 .s-icon-btn {