style(onboarding): ensure constant button position

A slight difference in height between the previous and next buttons
was causing the next button to appear to move when switching between
a page with or without the previous button.

Ensure that the previous and next buttons have a consistent height and
do not move when switching pages in the onboarding process.

app/components/Onboarding/FormContainer/FormContainer.scss

@@ -76,7 +76,7 @@
     .backButton {
       cursor: pointer;
       transition: all 0.25s;
-      padding: 10px 20px 10px 0;
+      padding: 8px 20px 8px 0;
       text-align: center;
 
       &:hover {