[REF] mail: stop using jQuery animate

Problem is that jQuery animate uses jQuery deferreds, so it may
happen in tests that we fold a thread window, and we have to wait
for 2 (?) nextTick to see that the window is actually folded.

We use a css transition instead, which produces the exact same
result.

addons/mail/static/src/js/thread_windows/abstract_thread_window.js

@@ -83,6 +83,8 @@ var AbstractThreadWindow = Widget.extend({
             displayStars: this.options.displayStars,
         });
 
+        // animate the (un)folding of thread windows
+        this.$el.css({transition: 'height ' + this.FOLD_ANIMATION_DURATION + 'ms linear'});
         if (this.isFolded()) {
             this.$el.css('height', this.HEIGHT_FOLDED);
         } else if (this.options.autofocus) {
@@ -319,24 +321,14 @@ var AbstractThreadWindow = Widget.extend({
                 this._focusInput();
             }
         }
-        this._animateFold();
+        var height = this.isFolded() ? this.HEIGHT_FOLDED : this.HEIGHT_OPEN;
+        this.$el.css({height: height});
     },
 
     //--------------------------------------------------------------------------
     // Private
     //--------------------------------------------------------------------------
 
-    /**
-     * Called when there is a change of the fold state of the thread window.
-     * This method animates the change of fold state of this thread window.
-     *
-     * @private
-     */
-    _animateFold: function () {
-        this.$el.animate({
-            height: this.isFolded() ? this.HEIGHT_FOLDED : this.HEIGHT_OPEN
-        }, this.FOLD_ANIMATION_DURATION);
-    },
     /**
      * Set the focus on the composer of the thread window. This operation is
      * ignored in mobile context.