Give maximum width & height constraint to tooltip boxes

Extreme values that go beyond the screen resolution are very likely to be misplaced. This is a simple workaround. A better solution would be to improve the code placing the tooltip and make it handle tooltips containing more content than they can safely display.

src/css/less/grafana.less

@@ -435,6 +435,9 @@ select.grafana-target-segment-input {
   background-color: rgb(58, 57, 57);
   border-radius: 5px;
   z-index: 9999;
+  max-width: 800px;
+  max-height: 600px;
+  overflow: hidden;
 }
 
 .tooltip.in {