Use calc() where divisions are involved

The division operator `/` from Sass is deprecated because `/` is used in
CSS for uses other than dividing numbers. That's why we were getting
many warnings like:

```
Deprecation Warning: Using / for division outside of calc() is
deprecated and will be removed in Dart Sass 2.0.0.

Recommendation: math.div($line-height, 2) or calc($line-height / 2)

More info and automated migrator: https://sass-lang.com/d/slash-div

margin-top: $line-height / 2;
```

Since using math.div makes the code harder to read and `calc` is
universally supported by all browsers (although the implementation in
Internet Explorer doesn't work in certain cases), we're using `calc`
when assigning the value to a CSS property.

However, we're also using divisions when assigning Sass variables, and
in those cases using `calc` is trickier because sometimes these
variables are used in other operations. We'll handle these cases in the
next commit.

app/assets/stylesheets/milestones.scss

@@ -30,7 +30,7 @@ $progress-bar-color:      #fea230;
     }
 
     .milestone-progress .row {
-      margin-bottom: $line-height / 2;
+      margin-bottom: calc(#{$line-height} / 2);
     }
   }
 }
@@ -68,7 +68,7 @@ $progress-bar-color:      #fea230;
   }
 
   .milestone-content {
-    padding: $line-height / 6 $line-height / 2;
+    padding: calc(#{$line-height} / 6) calc(#{$line-height} / 2);
     position: relative;
 
     @include breakpoint(medium) {
@@ -121,6 +121,6 @@ $progress-bar-color:      #fea230;
   @include background-with-text-contrast($budget);
   border-radius: rem-calc(4);
   display: inline-block;
-  margin-top: $line-height / 6;
-  padding: $line-height / 4 $line-height / 2;
+  margin-top: calc(#{$line-height} / 6);
+  padding: calc(#{$line-height} / 4) calc(#{$line-height} / 2);
 }