Improve display of failure rates

https://www.pivotaltracker.com/story/show/116052359

- add absolute numbers for failures
- make percentages accurate to 1 decimal place (50.0%)
- make error colour appear if failures go above 3.0%
- make error colour boolean (don’t interpolate between the colours)

app/assets/stylesheets/components/big-number.scss

@@ -25,21 +25,13 @@
     padding-bottom: 0;
   }
 
+  %big-number-status,
   .big-number-status {
 
+    @include core-19;
     display: block;
     background: $green;
-    position: relative;
-
-    &-error-percentage {
-      position: absolute;
-      top: 0;
-      left: 0;
-      width: 100%;
-      height: 100%;
-      background: $error-colour;
-      z-index: 1;
-    }
+    padding: 15px;
 
     a {
 
@@ -52,16 +44,11 @@
 
     }
 
-    .big-number {
-      @include bold-19;
-      position: relative;
-      z-index: 2;
-    }
-
-    .big-number-label {
-      display: inline;
-    }
+  }
 
+  .big-number-status-failing {
+    @extend %big-number-status;
+    background: $error-colour;
   }
 
 }