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/notification_item.scss

@@ -20,7 +20,7 @@
       font-size: $circle-icon-size;
       position: absolute;
       left: $notification-icon-size - rem-calc(5);
-      top: $menu-link-top-padding - $circle-icon-size / 2;
+      top: calc(#{$menu-link-top-padding} - #{$circle-icon-size} / 2);
 
       @include breakpoint(medium) {
         $menu-link-left-padding: rem-calc(16);