css: Size stream recipient box for small screens.

This allows the recipient box to take 100% of the available
horizontal space, up to 175px. The effect is that the
compose-box buttons are available to users at mobile scales
(viewports of 400px wide or less).

web/styles/compose.css

@@ -540,10 +540,11 @@ input.recipient_box {
 }
 
 #stream_message_recipient_topic.recipient_box {
+    width: 100%;
     /* This width roughly corresponds to how long of a topic can appear in
        the left sidebar with a single digit unread count without being
        cut off. */
-    width: 175px;
+    max-width: 175px;
 }
 
 #private_message_recipient.recipient_box {