Add a new join model to link summaries with users

So that changing the set of events does not result in not being able to find a generated highlight summary

script/migrations/backfill_weekly_highlights.rb

@@ -2,11 +2,12 @@
 
 require_relative "../config/environment"
 
-WEEKS_TO_BACKFILL = 10
+WEEKS_TO_BACKFILL = 3
 
 ActiveRecord::Base.logger = Logger.new(File::NULL)
 
 ApplicationRecord.with_each_tenant do |tenant|
+  PeriodHighlights.destroy_all
   WEEKS_TO_BACKFILL.times do |index|
     User.active.find_each do |user|
       user.generate_weekly_highlights(Time.current - index.weeks)