Try to match referral to appointment on single referral fetch

[brianseek]

We need to catch referrals that are already appointments. This could happen on the FE or the BE. If we do it on the FE, we should add a step to search for a matching referralId in the upcoming appointments payload. If it happens on the BE, vets-api should send a response that includes the existing appointment ID.

Regardless of who does this work, FE will need to redirect the user to either the appointment detail page or the complete page. Maybe we need some kind of a "you already did this" alert. Work with UX on the details for this scenario.