Don't add default margin to font awesome icons

We were removing the margin half of the time, and sometimes removing it
made us use `!important` rules.

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

@@ -23,6 +23,10 @@
   .preview-link {
     @include has-fa-icon(eye, regular);
     @include hollow-button;
+
+    &::before {
+      margin-right: $font-icon-margin;
+    }
   }
 
   .publish-link {

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

@@ -36,7 +36,6 @@
   &::after {
     bottom: $padding / 2;
     color: $white;
-    margin-right: 0;
     position: absolute;
     right: calc(#{$padding} + #{$quote-padding});
   }