Make main message area respond better to resizes.

Paradoxically, life is better if we specify a max-width rather
than a min-width!

(This has the side effect, though, of sometimes having our message
boxes, etc. be ~548px rather than 640px, but philosophically
it's kind of nice if that all scales nicely).

(imported from commit 7f41c147b727348a148197c74b299cb31636f83b)

zephyr/static/styles/zephyr.css

@@ -26,12 +26,11 @@ ul#sidebar {
 
 
 #application_area {
-    min-width: 670px;
 }
 
 .message_list {
+    max-width: 640px;
     background-color: aliceblue;
-    width: 640px;
     padding-left: 10px;
     padding-right: 20px;
 }