[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/_mixins.scss

@@ -348,7 +348,6 @@
 	display: inline-block;
 	font-family: 'symbolsfont';
 	margin-left: $interiorMarginSm;
-	vertical-align: top;
 }
 
 @mixin nice-textarea($bg: $colorBodyBg, $fg: $colorBodyFg) {