Use color-pick-contrast to get text colors

We were defining (for instance) white text against the `$brand`
background. That meant that, if somebody customized the `$brand` color
so it used a light color, they had to customize the text color as well
in order to guarantee proper contrast between text and background
colors.

So we're using `color-pick-contrast` instead, which means we don't have
to manually calculate whether white or black will be the color which
makes the text more readable.

app/assets/stylesheets/milestones.scss

@@ -23,7 +23,7 @@ $progress-bar-color:      #fea230;
     }
 
     .progress-meter-text {
-      color: #000;
+      color: color-pick-contrast($progress-bar-background);
       right: 12px;
       text-align: right;
       transform: translate(0%, -50%);
@@ -120,7 +120,7 @@ $progress-bar-color:      #fea230;
 .milestone-status {
   background: $budget;
   border-radius: rem-calc(4);
-  color: #fff;
+  color: color-pick-contrast($budget);
   display: inline-block;
   margin-top: $line-height / 6;
   padding: $line-height / 4 $line-height / 2;