We noticed there was a performance issue while browsing the SDG
Management section and when one of our tests started failing sometimes
because the request to the relations#index controller took too long.
The issue proved to be `SDG::Target#<=>`. This method calls `.goal` for
each target, meaning we were generating 169 database queries when
sorting all targets.
So we're comparing codes directly to minimize the number of database
queries and improve performance. Requests to the relations index take
now less than third of the time they used to take.
This is similar to what we do with investments, which belong to a heading
but also belong to a budget. In our case, the reason is we've been asked
to add local targets which belong to a goal but are not related to any
existing target.
Even though we're not implementing that case right now, we're adding the
relation so we don't have to add data migrations in the future.
