Fix N+1 query
Fix N+1 query

Currently the app is making a SQL query for the parent policies for
each policy.

Before

`Completed 200 OK in 640ms (Views: 545.5ms | ActiveRecord: 89.2ms)`

After

`Completed 200 OK in 705ms (Views: 656.2ms | ActiveRecord: 6.6ms)`

alphagov/policy-publisher,alphagov/policy-publisher,alphagov/policy-publisher,alphagov/policy-publisher