Enhancement: New branding/MDM parameter to send `login_hint` and `user` params

A new branding and MDM parameter has been created to decide if `login_hint` and `user` are
sent as parameters in the login request, so that a value is shown in the Username text field.

https://github.com/owncloud/android/issues/4288
https://github.com/owncloud/android/pull/4291
