Allow user to set their login when registering

Hi,

I added the possibility for a registering user to set its login.

Changes:

• Introduce registerDisplayLoginInput config variable to toggle the visibility of the login input.
• Use registerDisplayLoginInput to create a template variable DISPLAY_LOGIN_INPUT
• Create a login input in register.tpl
• Add the login param to the registerInfo object in Register.pm
• Use the provided login instead of calling computeLogin
• Make isLoginUsed method a public method in SAML and AD
• Add a isLoginUsed method to DEMO to make it compliant with the new API requirement
• Add default values for the variables in Attributes.pm

Extra changes:

• Introduce loginControl config variables to validate the login and validate the login in the registration flow in Register.pm
• Introduce registerLdapObjectClasses config variable to customize the objectClass in userCreation in LDAP.pm and AD.pm
• Introduce registerTransformNames to prevent first and last name transformation in userCreation

Needed changes (I would need guidance):

• Create a PE_LOGINALREADYEXISTS variable in Register.pm and a correct error message
• Add inputs in manager
• Write tests