Add option to open network login page

[chrisduerr]

Some networks, especially public ones, require opening a web page to confirm the ToS before being allowed to use the Internet. There should be a button in the settings that allows doing so.

NetworkManager recommends opening the browser page in a sandboxed browser.

See:

• https://developer-old.gnome.org/NetworkManager/stable/gdbus-org.freedesktop.NetworkManager.html#gdbus-method-org-freedesktop-NetworkManager.CheckConnectivity
• https://developer-old.gnome.org/NetworkManager/stable/nm-dbus-types.html#NMConnectivityState