Use multiplications instead of divisions in Sass variables

In the previous commit, we used the `calc` function when assiging CSS
properties in order to avoid 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($global-width, 2) or calc($global-width / 2)
```

In cases like dividing by two, there's a third alternative: multiplying
by 0.5. We're applying this principle to all variable assignments where
we were using divisions, since using the `calc` function here would
sometimes result in errors due to these variables being used in
arithmetical operations. We aren't using `math.div` because it makes the
code harder to read.

app/assets/stylesheets/admin/budgets/actions.scss

@@ -1,7 +1,7 @@
 .admin .budgets-actions {
   > * {
     $gap: $line-height;
-    $vertical-gap: $line-height / 2;
+    $vertical-gap: $line-height * 0.5;
     @include flex-with-gap($gap);
     align-items: center;
     flex-wrap: wrap;

app/assets/stylesheets/admin/budgets/drafting.scss

@@ -1,6 +1,6 @@
 .admin .drafting {
   $vertical-gap: nth($callout-margin, 3);
-  @include flex-with-gap($line-height / 2);
+  @include flex-with-gap($line-height * 0.5);
   align-items: flex-start;
   flex-wrap: wrap;
   margin-bottom: $line-height * 1.5;

app/assets/stylesheets/admin/budgets/help.scss

@@ -1,7 +1,7 @@
 .admin .budgets-help {
-  $padding: $line-height / 2;
+  $padding: $line-height * 0.5;
   $quote-size: 1em;
-  $quote-padding: 2 * $quote-size / 5;
+  $quote-padding: $quote-size * 0.4;
   $quote-width: $quote-size + 2 * $quote-padding;
 
   @include has-fa-icon(quote-right, solid, after);