Add CORS support

The sso service endpoint should send the proper CORS headers and
respond to OPTIONS (for CORS prefetches) so that XmlHttpRequests on
authenticated sites can (partially) work.
4 jobs for master in 4 minutes and 32 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build Pkgsrc
passed #9173
build_pkgsrc:stretch

00:00:18

 
  Build Pkg
passed #9174
amd64
build_pkg:stretch:amd64

00:01:01

passed #9175
arm64
build_pkg:stretch:arm64

00:02:57

 
  Upload Pkg
passed #9176
upload_pkg

00:01:13