pills: Explicitly declare height of input pill as 20px.

Adding the 20*20 image inside the pill caused a minor increase in
pill height. Making the image 19*19 causes some increase in the height
under different zoom conditions. I'm not sure about the reason behind
this, so this can be counted as a hack.

static/styles/input_pill.scss

@@ -14,6 +14,7 @@
     position: relative;
     display: inline-block;
 
+    height: 20px;
     padding: 0px 18px 0px 3px;
     margin: 1px 1px;