keycloak: Switch to new account management page

To enable #28 (closed) we need the new accounts management page. The so-called account2 feature has been integrated in keycloak since 2017 and does receive regular updates. I think it's safe to switch to this. Advantages: 1) It looks much nicer 2) It allows us to eventually use webauthn.

It appears like the old account management page is not going to allow users to manage their own webauthn devices.

The changes have already been rolled out and are available for review on our Keycloak.