Add an "Advanced Settings" option to the gpodder sign in. If selected, a fully
qualified URL must be specified as the gpodder base. Upon successful login, the
URL is saved along with username and password. If advanced settings are not
selected, an empty URL is stored and the default will be used.
It's not necessary for the PodcastSettingsPage class to have knowledge of
GPodderSync's login implementation. Handling the network reply in a single
location sightly simplifies the code. It also makes the handling order
more deterministic.