Check if username/email is taken in registration

api/src/main/kotlin/com/wbrawner/twigs/UserRoutes.kt

@@ -49,12 +49,24 @@ fun Application.userRoutes(
                     errorResponse(HttpStatusCode.BadRequest, "Password must not be null or blank")
                     return@post
                 }
+                val existingUser = userRepository.findAll(nameOrEmail = request.username).firstOrNull()
+                        ?: request.email?.let {
+                            return@let if (it.isBlank()) {
+                                null
+                            } else {
+                                userRepository.findAll(nameOrEmail = it).firstOrNull()
+                            }
+                        }
+                existingUser?.let {
+                    errorResponse(HttpStatusCode.BadRequest, "Username or email already taken")
+                    return@post
+                }
                 call.respond(
                         userRepository.save(
                                 User(
                                         name = request.username,
                                         password = request.password.hash(),
-                            email = request.email
+                                        email = if (request.email.isNullOrBlank()) null else request.email
                                 )
                         ).asResponse()
                 )