Improve UX of login page when multiple idp's are delcared

A reoccurring issue we have with clients who have setup either manual auth + saml auth, or similar with a mix of internal and external authentication is that the student get confused about which login box to use. We often do a fair bit of repetitive custom work on the login page, beyond the expected theming work, to make this nicer.

One minor feature which would go a long way is a setting so that when the student hits the login page all login methods are given equal footing, and they choose 'manual' or 'internal' and then login. This could be either an intermediate chooser page, or could just even just be some JS hide / show stuff.

I just had a PR on the auth_saml2 plugin but I feel this should be better in core rather than in potentially each auth plugin that implements loginpage_idp_list()

https://github.com/CatalystIT-AU/moodle-auth_saml2/issues/20