stats: Don't set display to none in alert class.

The alert class is used only by one element so this style
can be applied directly to the element instead.

static/styles/portico/stats.scss

@@ -108,10 +108,6 @@ p {
     float: right;
 }
 
-.alert {
-    display: none;
-}
-
 .button {
     font-family: 'Source Sans Pro', 'Helvetica Neue', sans-serif !important;
     border: none;
@@ -185,6 +181,10 @@ p {
     position: relative;
 }
 
+#id_stats_errors {
+    display: none;
+}
+
 #users_hover_info,
 #hoverinfo {
     display: none;