XNAT-4387 Made auth methods fully customizable via plugins. If none are...

XNAT-4387 Made auth methods fully customizable via plugins. If none are specified, it will default to db authentication, but if any are specified, only the specified ones will show up. I also added support for an order property to control the order of the methods in the dropdown, as well as the order in which they are checked when users attempt authentication. I see this plugin-based way of configuring providers remaining as an alternative to AdminUI configuration once that is implemented.