Return consistent type from isHidden

Return boolean instead of number in all cases.

src/sidebar/components/moderation-banner.js

@@ -17,7 +17,7 @@ function ModerationBannerController(annotationUI, flash, store) {
   this.isHidden = function () {
     var moderation = self.annotation.moderation;
     if (!moderation) {
-      return 0;
+      return false;
     }
     return moderation.is_hidden;
   };