add icon for Archive folder, ref #307

css/mail.css

@@ -602,6 +602,9 @@
 .icon-sent {
 	background-image: url('../img/sent.svg');
 }
+.icon-archive {
+	background-image: url('../img/archive.svg');
+}
 .icon-junk {
 	background-image: url('../img/junk.svg');
 }

img/archive.svg

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns="http://www.w3.org/2000/svg" height="16" width="16" version="1.1" xmlns:cc="http://creativecommons.org/ns#" xmlns:dc="http://purl.org/dc/elements/1.1/">
+ <path opacity=".5" d="m4 2-4 6v5h16v-5l-4-6h-8zm1 1h6l3 5h-4v3h-4v-3h-4l3-5z"/>
+</svg>