Draft: Resolve "Allow adding students/staff using email/student number/name"

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Closes #99 (closed)

src/main/java/nl/tudelft/queue/controller/EditionController.java

@@ -464,14 +464,14 @@ public class EditionController {
 		}
 
 		try {
-			PersonDetailsDTO person = pApi.getPersonByUsername(dto.getUsername()).block();
+			PersonDetailsDTO person = pApi.getPersonByIdentifier(dto.getUserIdentifier()).block();
 			return checkDemoteThenAddSingle(person, edition, redirectAttributes, create, model);
 
 		} catch (WebClientResponseException e) {
 			redirectAttributes.addFlashAttribute("error",
-					String.format("Person with username: %s cannot be found. " +
-							"Make sure this person has logged into our system before",
-							dto.getUsername()));
+					String.format("Person with the user identifier: %s cannot be found. " +
+							"Make sure this person has logged in our system before.",
+							dto.getUserIdentifier()));
 			return "redirect:/edition/" + editionId + "/participants/create";
 		}
 	}