Apply rubocop Rails/FindBy rule everywhere

We didn't detect these cases because by default the rule only searches
for offenses in `app/models/`.

app/controllers/admin/homepage_controller.rb

@@ -13,7 +13,7 @@ class Admin::HomepageController < Admin::BaseController
     end
 
     def load_recommendations
-      @recommendations = Setting.where(key: "feature.user.recommendations").first
+      @recommendations = Setting.find_by(key: "feature.user.recommendations")
     end
 
     def load_cards