user websites: Include in Webserver homepage configuration

The possibility to have a user's website as a choice for webserver homepage configuration has popped up in the forum. See https://discuss.freedombox.org/t/user-websites-to-be-selectable-as-webserver-home-page/1175

Usual Freedomboxes should have a small enough number of users to just populate the dropdown. But we should think of a more robust solution for the case of a Freedombox with a large number of users.

One intermediate solution might be to check user sites for existing index.html files on them and only populate these in the list.

Over a certain number this feature might not make sense anymore: If there's a lot of user websites, why promoting a particular one over the others? A system-wide index page would make more sense in such a case.

And in the scant such cases where it still is needed, an admin should be able to tweek the hardcoded threshold or issue a manual Apache redirection).