Fix button line-height issue (#94)

Math button labels weren't properly aligned; neither were the buttons
themselves in the math tools preference description.

Regression introduced in #82.

src/stylesheets/editing-tools.scss

@@ -31,6 +31,9 @@
         height: 23px;
         box-sizing: border-box;
         margin: 0 1px 0 0;
+        * {
+            line-height: 0;
+        }
     }
 
     .#{getGlobal("SITE.CLASS.button")}:last-of-type {