Initially, I had opted to use getReference() to avoid querying the
database to get the segment entities as all we need is the segment ID.
But I hadn't realized that this could cause problems if a segment that
is saved in the settings in the option woocommerce.segments is deleted.
Using findBy() here protects against this problem as, if the segment
doesn't exist anymore, it won't be returned.
[MAILPOET-4365]
Besides Subscription::subscribeOnCheckout(), this commit also replaces
Paris models with Doctrine entities in all the methods that are called
by Subscription::subscribeOnCheckout().
[MAILPOET-4365]
