Restores github.com/duo-labs/webauthn to commit 03696f3, which still
builds with Go 1.15 as shipped in Debian stable. Commits past 41e0a04
require Go 1.18.

Provides SQL-related helpers, mostly targeted to a simple sqlite
embedded database.

The ProxyHandler apparently assumes that you're running a gorilla.Mux
for request routing, as it sets the URL.Scheme field of the incoming
request - which is normally unset. This has the consequence of messing
up all sorts of things, including redirects, because the other
fields (like Host) of the request URL are still going to be unset.