Fix for issue #963. (#1357)

iOS <button> element's minimum width is dependant on its font-size, so the .delete width wasn't able to be as small as it's height, resulting in one arm of the X being longer than the other. font-size: 0 does the trick.

sass/utilities/mixins.sass

@@ -42,7 +42,7 @@
   display: inline-block
   flex-grow: 0
   flex-shrink: 0
-  font-size: $size-normal
+  font-size: 0
   height: 20px
   max-height: 20px
   max-width: 20px