Add signin page.

backend/codiki-exposition/src/main/java/org/codiki/exposition/configuration/security/SecurityConfiguration.java

@@ -54,7 +54,8 @@ public class SecurityConfiguration {
                 .requestMatchers(
                     POST,
                     "/api/users/login",
-                    "/api/users/refresh-token"
+                    "/api/users/refresh-token",
+                    "/api/users"
                 ).permitAll()
                 .requestMatchers(
                     POST,