Many older devices do not support the Let's Encrypt CA, for various
reasons. This causes connection issues for sites using Let's Encrypt to
support HTTPS connections. This commit adds a hack that can be enabled
with a switch in callbacks.js to try to detect when the user's browser
does not trust the certificate and permit the user to connect to an
insecure endpoint instead.
Unfortunately, the AJAX API does not allow to distinguish between *why*
a request fails, so the best we can do is detect that the HTTPS request
failed, try to make a request over plain HTTP, and if it works, assume
the HTTPS request failed due to a certificate error. It's not 100%
foolproof since the HTTPS endpoint could just be down for some reason,
but it should work well enough in most cases.
Closes#602
6e416fea8a