Adds JWT secret, turns on Swagger UI, and cleans up refresh token response

src/main/kotlin/net/halfbinary/scavengerhuntapi/config/JwtUtil.kt

@@ -7,11 +7,12 @@ import org.springframework.beans.factory.annotation.Value
 import org.springframework.stereotype.Component
 import java.util.Date
 import javax.crypto.SecretKey
+import javax.crypto.spec.SecretKeySpec
 
 @Component
 class JwtUtil {
     @Value($$"${jwt.secret}")
-    private val jwtSecret: String? = null
+    private val jwtSecret: String = ""
 
     @Value($$"${jwt.expiration}")
     private val jwtExpirationMs = 0
@@ -22,7 +23,7 @@ class JwtUtil {
     // preventing the repeated creation of the key and enhancing performance
     @PostConstruct
     fun init() {
-        this.key = Jwts.SIG.HS256.key().build()
+        this.key = SecretKeySpec(jwtSecret.toByteArray(Charsets.UTF_8), "HmacSHA256")
     }
 
     // Generate JWT token