Feat/navigate to signup if email does not exist (#540)

* Add userExists route

* Fix demo mode for login

* Improve sign in/up flow

* Remove redundant password length constraint

* Fix test

front/src/modules/auth/services/index.ts

@@ -1 +1,2 @@
+export * from './select';
 export * from './update';

front/src/modules/auth/services/select.ts

@@ -0,0 +1,9 @@
+import { gql } from '@apollo/client';
+
+export const CHECK_USER_EXISTS = gql`
+  query CheckUserExists($email: String!) {
+    checkUserExists(email: $email) {
+      exists
+    }
+  }
+`;