move the #vouch irc channel to libera.chat

vouch#375 use correct error codes, general cleanup

ignore rsa keys created for testing

Merge branch 'pr/simongottschlag/320'

bump to go version 1.16

don't log the oauth client creds, even in debug

fix vouch#368 build on FreeBSD and Linux

vouch#344 formatting, add README entry

fix vouch#361 with go modules, upgrade Docker to go 1.15

vouch#354 clarify audience (aud) check

* move claims.Sites into claims.Audience
* move check Site to check Audience
* general cleanup
* rename CreateUserTokenString to NewVPJWT