[Frontend] CSS hooks for staleness

WTD-1172
New .scss file for staleness
Added CSS hooks in templates for staleness

platform/features/plot/res/templates/plot.html

@@ -9,7 +9,7 @@
             <span
 	            class='plot-legend-item'
                 ng-repeat="telemetryObject in subplot.getTelemetryObjects()"
-                ng-class="{ 's-limit-lwr': $index < 1, 's-limit-yellow' : 1 }"
+                ng-class="{ 's-limit-upr s-limit-red': $index < 1, 's-stale': $index%2==0 }"
 	            >
                 <span class='plot-color-swatch'
                       ng-style="{ 'background-color': plot.getColor($index) }">