Task to drop all ahoy events

The ChangeVisitIdTypeOfAhoyEventsToInteger from 364be5541 is not very well done.
You can not change the schema without changing possibly existing data.
Ahoy events used some kind of string as `visit_id`. Changing this column
to integrer will just barf up `Mysql2::Error: Truncated incorrect INTEGER value`.

So we need a way for people who are in this situation now to get out of it.
Luckily we completely dropped ahoy events a while ago so we can just delete them.

lib/tasks/data.rake

@@ -17,6 +17,14 @@ namespace :data do
     nullify_attribute(events_room, 'room_id')
   end
 
+  desc 'Drop all ahoy events'
+  task drop_all_ahoy_events: :environment do
+    class TmpAhoy < ActiveRecord::Base
+      self.table_name = 'ahoy_events'
+    end
+    TmpAhoy.delete_all
+  end
+
   def nullify_attribute(collection, attribute)
     puts "Will nullify #{attribute} in #{ActionController::Base.helpers.pluralize(collection.length, 'event')}."
     if collection.any?