Fix tooltips not visible for NVD3 charts on Firefox (apache#7822) (ap…

…ache#7929)

This bug was introduced by apache#7102

Using `position:absolute` on body gives `document.documentElement` a height of 0 which propagates to `clientHeight` on Firefox. This leads to a wrong calculation of the tooltip position in NVD3.

The solution proposed here is to use `min-height: 100vh` instead of the current technique for stretching the body element to the full window height, thus keeping body and html together.

superset/assets/stylesheets/less/index.less

@@ -25,11 +25,7 @@
 @stroke-primary:  @brand-primary;
 
 body {
-  position: absolute;
-  top: 0;
-  right: 0;
-  bottom: 0;
-  left: 0;
+  min-height: 100vh;
   display: flex;
   flex-direction: column;
 }