Make input buttons and link buttons the same height in Firefox

Set min-height on the input buttons to work around Firefox ignoring the
line height: https://bugzilla.mozilla.org/show_bug.cgi?id=697451#c13.

app/assets/stylesheets/common.css.scss

@@ -1706,6 +1706,7 @@ a.button {
   display: inline-block;
   line-height: 20px;
   padding: $lineheight/4 $lineheight/2;
+  min-height: 20px + $lineheight/2;
   min-width: 120px;
   margin: 0 0 $lineheight/2 0;
   color: white;