It might be that it's shown in response to a failed XEP-156 lookup
because the user mistyped.
In which case the user can just correct the JID and try again, without
entering a connection URL.
Render the form based on `api.settings` instead of its own model.
When the login form is submitted, save the JID, password and connection
URL to `api.settings`.
Set the `service` on the Strophe connection object just before
connecting for the first time, otherwise a user supplied URL (via the
login form) is never used.
New API setting: show_connection_url_input
