Fix "done"/"saved" statuses to not block clicks when they disappear.

Also actually hide them on IE8,9.
(part of collateral damage I inflicted dealing with #56)

Tested on Chrome and Firefox, will test more before deploying.
This really could use automated testing, but it's rather tricky :-(

http://stackoverflow.com/a/10352525/239657 and
http://www.taccgl.org/blog/css-transition-visibility.html
have good explanations why `visibility` transition does the right thing.

I discovered that I had to set `#status_container{ visibility: hidden }`
or it wouldn't work on Chrome; this also avoids #status_container
from blocking clicks, so reverting the less elegant `height: 0`.

I wasn't sure if I should also set `pointer-events: none`.  It makes sure
you can click through the status even when it's visible, but I'm not sure
that's a feature, it doesn't work on IE<11, and would mysteriously prevent
adding a hover tooltip to statuses.

mathdown.css

@@ -15,24 +15,27 @@
 
 /* Horizontally center http://stackoverflow.com/a/10352525/239657 */
 #status_container {
-  position: fixed; top: 1ex; left: 0; right: 0; height: 0;
+  position: fixed; top: 1ex; left: 0; right: 0;
   text-align: center;
+  visibility: hidden;  /* allow clicking through */
 }
 #status {
   display: inline-block;
   padding: 0 0.5em; border-radius: 1ex;
   color: black; background-color: yellow;
   font-size: 150%; font-style: italic;
-  opacity: 0;
-  -webkit-transition: opacity 0.3s ease;
-  transition: opacity 0.3s ease;  /* fast fade out */
+  /* Without a class, hide the status.
+     Make sure stuff underneath is clickable (see http://stackoverflow.com/a/21335440/239657). */
+  opacity: 0; visibility: hidden;
+  -webkit-transition: all 0.3s ease;
+  transition: all 0.3s ease;  /* fast fade out */
 }
 #status.info {
   /* Explanation for http://cubic-bezier.com/#0,.3,.85,0 curve:
      quickly appear a tiny bit (10% at 5% time), then fade in slowly (50% at 80% time). */
-  opacity: 0.8;
-  -webkit-transition: opacity 2s cubic-bezier(0,.3,.85,0);
-  transition: opacity 2s cubic-bezier(0,.3,.85,0);  /* slow appear */
+  opacity: 0.8; visibility: visible;
+  -webkit-transition: all 2s cubic-bezier(0,.3,.85,0);
+  transition: all 2s cubic-bezier(0,.3,.85,0);  /* slow appear */
 }
 
 @media print { #header { display: none; } }