Size emoji more responsively.

Now when the font increases, so will the size of the emoji.  (1.4em
seems to be 20px at our default settings, so this doesn't change the
size of the emoji for any of our existing people).

(imported from commit edb0b590f00bfbad0355a41b1f995335cf0e9e07)

zephyr/static/styles/zephyr.css

@@ -1024,5 +1024,5 @@ table.floating_recipient {
 }
 
 .emoji {
-    height: 20px;
+    height: 1.4em;
 }