[Frontend] Styling Export button

fixes #973
- Done;
- Styling for Export button and tabular view
area in layout frame context;
- Export button in frame context now
hidden until user hovers over tabular
view area in frame, includes animated
transition;
- Normalized line-height on button and
menu elements in frame context;
- Layout/markup/SASS for historical and
RT tabular view modified;
- Converted imagery.html layout
to use flexbox;

platform/commonUI/general/res/sass/user-environ/_frame.scss

@@ -54,7 +54,8 @@
 			height: $ohH;
 			line-height: $ohH;
 			padding: 0 $interiorMargin;
-			> span {
+			> span,
+            &:before {
 				font-size: 0.65rem;
 			}
 		}