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/admin/tenants/form.scss

@@ -6,10 +6,10 @@
 
   .radio-and-label {
     display: flex;
-    margin-bottom: $line-height / 3;
+    margin-bottom: calc(#{$line-height} / 3);
 
     &:last-of-type {
-      margin-bottom: $line-height * 2 / 3;
+      margin-bottom: calc(#{$line-height * 2} / 3);
     }
 
     input {