Add min/max username validation from client info settings

[cristiandouce]

After the introduction of the new feature for configuring username validation min/max values at Auth0 we required to handle those modifications within the Lock UI. Therefore:

1. If the validation settings is defined within the client info object, use those parameters (configured from the Dashboard)
2. If validation settings is missing from within the client info settings, defaults to the current values {min: 1, max: 15}.
3. No validations are performed for enterprise connections, as it was the previous behavior.