Make it easy to open the Unsafe Browser only from Tor Connection (when a captive portal is detected)

• Steps
  • Have a LAN Browser that can only access RFC 1918 IP ranges
  • Remove the graphical launcher for the Unsafe Browser (I suppose we can do this once we've done #1) ⇒ the only ways to start it are Tor Connection (legit) or sudo unsafe-browser in a Terminal (that's OK, power users will always find ways to shoot themselves in the foot)
  • Design how we display troubleshooting options when Tor fails to bootstrap after we've detected a captive portal (ideally we would not even try to bootstrap, but that's a bigger topic, better suited for #5785 (closed))
  • Hide donation banners in Unsafe Browser using .hide-in-unsafe-browser in Ikiwiki templates (@sajolida knows what this is about)
• Blocked by
  • Open the Unsafe Browser from Tor Connection (#18339 - closed) (fixed by !838 (merged))
  • Detect captive portals (#5785 - closed) (we already detect captive portals actually, so the real blocker is making the "we've detected a captive portal" info available to the relevant code in Tor Connection, so it can adjust the troubleshooting screen accordingly)