user should be notified when they have connected to a wifi network with a captive portal

captive portals often require additional (manual) action in order to establish connectivity. I think it would be great to show a notification to the user when they connect to one so that they know some additional work is required.

This seems to be due to us not configuring the connectivity check in NM. Some example config:

Fedora:

# Enable connectivity checking for NetworkManager.
# See `man NetworkManager.conf`.
#
# Note that connectivity checking works badly with rp_filter set to
# strict. Check "/proc/sys/net/ipv4/conf/*/rp_filter".
[connectivity]
enabled=true
uri=http://fedoraproject.org/static/hotspot.txt
response=OK
interval=300

And from Arch:

[connectivity]
uri=http://ping.archlinux.org/nm-check.txt

We might need some infra for this, and consider privacy concerns with configuring NM to connect OOTB for this (what headers, info does it send? obviously IP, but what else)