Don't rely on existence of /profile/card to block sign-up.

In single-user mode, you can register once.
If you try to register a second time, https://github.com/solid/node-solid-server/blob/master/lib/models/account-manager.js#L111 will notice that ./data/profile/card$.ttl already exists, and respond with a 400.

However, if you manually delete that file, then that means that if someone goes to the Register page, they can take over control of your pod.

For multi-user mode, this would presumably work too, but then you need to "guess" the username for which the profile doc is missing, so it's less likely to occur.

We could change the check for user existence, for instance, (also) check if an oidc user exists, or if a data folder exists at all.