Check External WebID when creating a new account

[samuelmr]

When a user registers to Solid server she may enter an external WebId.

If the user enters an external WebId, the server should at least check that the WebId exists. If not, a warning should be shown to the user with the possibility to correct the WebId.

A bonus feature would be to check if the WebId document can be used for login (e.g. it contains cert:key).

An alternative approach would be to allow account creation either with username and password or using WebId. If a user wants to user WebId, she would need to authenticate with her WebId, not just type the URL.