Add ad-hoc + Captive Portal workflow for network connectivity

[aniongithub]

Intended workflow

1. On startup, check for network connectivity (use secret with reasonable defaults for host to ping - is there a better way?). If found, exit this setup script. We're good.
2. If not found, scan for all SSIDs and store this list.
3. Set up an ad-hoc network with the hostname of the device and no password
4. Blink the on-board LED using this method in a clear pattern that cannot be confused with flash reading, like a 1 Hz blink.
5. Use the Captive Portal method to display a webpage with the previously scanned SSIDs. See OpenNDS
6. When the user selects one of them, prompt them to enter any information required to connect to that SSID (password, country)
7. Write wpa_supplicant.conf with this information.
8. Reboot and go to step 1

[aniongithub]

Relevant links
https://www.raspberrypi.org/documentation/configuration/wireless/access-point-routed.md
https://www.raspberrypi.org/forums/viewtopic.php?t=270776 (fix this script to better handle dhcpcd.conf without using line numbers).