Chang mail icon to the material design icon

Signed-off-by: greta <gretadoci@gmail.com>

core/img/actions/mail.svg

@@ -1 +1 @@
-<svg xmlns="http://www.w3.org/2000/svg" version="1.1" height="16" width="16" viewBox="0 0 16 16"><path d="m0.89 3c-0.4924 0-0.89 0.4-0.89 0.89v8.2202c0 0.493 0.4 0.89 0.89 0.89h14.22c0.492 0 0.89-0.4 0.89-0.89v-8.2202c0-0.4924-0.4-0.89-0.89-0.89zm0.75 1.0278 6.0827 6.0817h0.52773l6.1102-6.0817 0.611 0.6109-3.6384 3.6934 2.75 2.8047-0.61102 0.61092-2.8052-2.8047-2.0275 2.0549h-1.2776l-2.0271-2.0553-2.8053 2.8323-0.6111-0.639 2.7774-2.8046-3.666-3.6932z"/></svg>
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24"><path d="M20,8L12,13L4,8V6L12,11L20,6M20,4H4C2.89,4 2,4.89 2,6V18A2,2 0 0,0 4,20H20A2,2 0 0,0 22,18V6C22,4.89 21.1,4 20,4Z" /></svg>