Don't show sender email in skinny windows

It can easily cost an entire line, and the information is available by
clicking on the sender's name.  Plus on a phone, you can't hover
anyways.

Annoyingly, this tends to put the popover partly off the window, but
there must be a way to fix that separately.

(imported from commit 19334cb067981b323e300245654c83c8e545fb2e)

zephyr/static/styles/zephyr.css

@@ -206,6 +206,11 @@ td.pointer {
        region of 'hover-ability' between fullname and email. */
     padding-left: 5px;
 }
+@media (max-width: 480px) {
+    .sender_email {
+        display: none;
+    }
+}
 
 .message_label_clickable:hover {
     cursor: pointer;