Allow disabling username suggestions for some login options

For example, with Google OAuth you get a somewhat ugly name in the form of Firstname_Lastname_Nick and a custom OAuth service we use sends md5(email)[:20] as the username for certain users (which is obviously awful, and even with the max username length set to 19 there’s a risk of people keeping this nonsense by just removing the last char if that’s not already done automatically…)

If you’re using the oauth2 basic plugin, it should work just fine if you don’t provide a username json path.