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/account/permissions_list.scss

@@ -5,7 +5,7 @@
 
   li {
     font-size: $small-font-size;
-    margin-bottom: $line-height / 2;
+    margin-bottom: calc(#{$line-height} / 2);
 
     &::before {
       font-size: 0.9em;

app/assets/stylesheets/account/verify-account.scss

@@ -1,5 +1,5 @@
 .verify-account {
-  padding-right: $line-height / 2;
+  padding-right: calc(#{$line-height} / 2);
 
   .already-verified {
     @include has-fa-icon(check, solid);