Fix line height for buttons in Firefox 30

This is pretty ugly, but it's the best solution I have at the
current time. Seems to fix Firefox 30 without breaking Chrome
or Opera and doesn't make IE8 any worse.

Fixes #756

app/assets/stylesheets/common.css.scss

@@ -1931,7 +1931,6 @@ a.button {
   cursor: pointer;
   border: 0;
   display: inline-block;
-  line-height: 20px;
   padding: $lineheight/4 $lineheight/2;
   min-height: 20px + $lineheight/2;
   min-width: 120px;
@@ -1962,6 +1961,16 @@ a.button {
   }
 }
 
+input[type="button"],
+input[type="submit"],
+input[type="reset"] {
+  line-height: normal;
+}
+
+a.button {
+  line-height: 20px;
+}
+
 .buttons {
   min-width: 200px;
   input[type="submit"],