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/stats.scss

@@ -16,7 +16,7 @@
     }
 
     ul {
-      margin-left: $line-height / 2;
+      margin-left: calc(#{$line-height} / 2);
       margin-top: 0;
       padding: 0;
     }
@@ -37,7 +37,7 @@
 
     &::before {
       display: inline-block;
-      margin-right: $line-height / 2;
+      margin-right: calc(#{$line-height} / 2);
       vertical-align: top;
     }
 
@@ -99,7 +99,7 @@
   .progress {
     background: #ebf0f4;
     border-radius: rem-calc(20);
-    height: $line-height / 2;
+    height: calc(#{$line-height} / 2);
   }
 
   .progress-meter {