FIX: Add migration to set correct redemption_count (#12491)

Redeeming email invites did not increase the redemption_count which let
those invites in a weird state were they were both pending and redeemed.

db/migrate/20210323142518_update_invites_redemption_count.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+class UpdateInvitesRedemptionCount < ActiveRecord::Migration[6.0]
+  def change
+    execute <<~SQL
+      WITH invite_counts AS (
+        SELECT invite_id, COUNT(*) count
+        FROM invited_users
+        GROUP BY invite_id
+      )
+      UPDATE invites
+      SET redemption_count = GREATEST(redemption_count, count)
+      FROM invite_counts
+      WHERE invites.id = invite_counts.invite_id
+    SQL
+  end
+end