Adding England flag: Issue #1549 removed the alias uk-england, but England was never added back.

dist/components/flag.css

@@ -301,6 +301,10 @@ i.flag.eh:before,
 i.flag.western.sahara:before {
   background-position: 0px -1664px;
 }
+i.flag.en:before,
+i.flag.england:before {
+  background-position: 0px -1690px;
+}
 i.flag.er:before,
 i.flag.eritrea:before {
   background-position: 0px -1716px;