Fix selected-message glitch on composewindow by removing animation.

The issue with the animation is that it removes the composebox div
when it's done -- or more relevantly, it "adds" it when the composebox
appears, which causes some DOM elements to get reshuffled slightly
which causes some jitter.

(Similar to what was happening with the email addresses earlier.)

So instead of using display:none, we play with visibility:hidden,
which causes the thing not to show up, but doesn't cause it to
lose its place in the DOM.

(imported from commit a18dbdcd1784b2b54436d48d8425d5fdc8dfbba4)

zephyr/static/js/compose.js

@@ -1,7 +1,7 @@
 var status_classes = 'alert-error alert-success alert-info';
 
 function show_compose(tabname, focus_area) {
-    $('.zephyr_compose').slideDown(100);
+    $('#zephyr_compose').css({visibility: "visible"});
     $('#zephyr-type-tabs a[href="#' + tabname + '-message"]').tab('show');
     focus_area.focus();
     focus_area.select();
@@ -9,7 +9,7 @@ function show_compose(tabname, focus_area) {
 
 function hide_compose() {
     $('input, textarea, button').blur();
-    $('.zephyr_compose').slideUp(100);
+    $('#zephyr_compose').css({visibility: "hidden"});
 }
 
 function clear_compose_box() {

zephyr/static/styles/zephyr.css

@@ -195,8 +195,8 @@ img.profile_picture {
     height: 30px;
 }
 
-.zephyr_compose {
-    display: none;
+#zephyr_compose {
+    visibility: hidden;
     position: fixed;
     bottom: 0px;
     background: white;