Reduce or stop WiFi scanning when not being used

[fhunleth]

It turns out to be convenient when debugging to put a device into AP mode with the wizard and ssh into it. This works great except for one flaw: The WiFi scan that occurs every 20 seconds introduces an annoying hiccup. See:

https://github.com/nerves-networking/vintage_net_wizard/blob/master/lib/vintage_net_wizard/backend/default.ex#L174

Fixes to this could be to disable scanning until someone loads a web page or scan at a low frequency (once every 5 minutes?) until someone loads the web page and then scan more regularly. Or maybe just do this if the most recently loaded web page is the access point scanning one.

Whatever is chosen, the user experience of WiFi configuration is still the most important part, but reducing scanning to make the debugging experience better would be really nice. As an aside, it really seems like the scanning should cause hiccups when loading webpages too, but I haven't noticed those.