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/relationable/related_content_list.scss

@@ -5,7 +5,7 @@
   li {
     border-bottom: 1px solid $border;
     margin-bottom: 0 !important;
-    padding: $line-height / 2;
+    padding: calc(#{$line-height} / 2);
 
     @include breakpoint(medium) {
       display: flex;
@@ -34,6 +34,6 @@
   }
 
   .flag {
-    margin-top: $line-height / 2;
+    margin-top: calc(#{$line-height} / 2);
   }
 }