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/budgets_wizard/creation_timeline.scss

@@ -9,7 +9,7 @@
     display: inline-block;
     font-size: $small-font-size;
     font-weight: bold;
-    padding: $line-height / 2 $line-height * 3 0;
+    padding: calc(#{$line-height} / 2) $line-height * 3 0;
     text-transform: uppercase;
 
     &::before {
@@ -17,7 +17,7 @@
       border-radius: 50%;
       content: "";
       height: 20px;
-      margin-left: $line-height / 2;
+      margin-left: calc(#{$line-height} / 2);
       position: absolute;
       top: -8px;
       width: 20px;