UX: Check whether we might be on a captive portal if Tor fails to connect

This is an outcome of the 1st Tails usability testing session.

it would be nice to have a little message saying “it might be that you’re on a captive portal network. you might want to open the unsafe browser to connect to it” if Tor fails to synchronize after some time.

Related issues

Is duplicate of #5785

Original created by @u on 7448 (Redmine)

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