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/application_controller.rb

@@ -98,7 +98,7 @@ class ApplicationController < ActionController::Base
 
     def track_email_campaign
       if params[:track_id]
-        campaign = Campaign.where(track_id: params[:track_id]).first
+        campaign = Campaign.find_by(track_id: params[:track_id])
         ahoy.track campaign.name if campaign.present?
       end
     end