Refactor and code cleaning on Authentication

[paul0vinicius]

• Correct name 'snackbarsService' -> 'snackbarService' and update all references;
• Move navbar.service.ts directory from src/app/navbar/navbar.service.ts ->src/app/services/navbar.service and update all the references;
• Refactor general functions on Requests, Users and App folders;
• Create constants correctly on those folders.