use button for the 'New message' button and fix its styling

css/mail.css

@@ -391,7 +391,6 @@
 
 /* editor */
 #mail-new-message-fixed {
-	width: 241px !important;
 	z-index: 111; /* navigation menu is 110 ;-) */
 	background-color: rgba(255, 255, 255, .75);
 	padding: 10px;

js/templates/newmessage.html

@@ -1,4 +1,3 @@
-<input type="button"
+<button type="button"
        id="mail_new_message"
-       class="icon-add"
-       value="{{ t 'New message' }}">
+       class="icon-add">{{ t 'New message' }}</button>