fix($animate): remove the need to add display:block!important for ngShow/ngHide

Since ngShow/ngHide animations add and remove the .ng-hide class, having to remember
to write display:block on your own is a hassle and leads to problematic animation
code. This fix places a default on the animation for you instead.

Closes #3813

css/angular.css

@@ -9,3 +9,9 @@
 ng\:form {
   display: block;
 }
+
+/* show the element during a show/hide animation when the
+ * animation is ongoing, but the .ng-hide class is active */
+.ng-hide-add-active, .ng-hide-remove {
+  display: block!important;
+}