Add templating for newsletter resources

Will set common attributes (status, shard, etc) on newly created
newsletters, just as we do for mailing lists.
3 jobs for add_newsletters in 5 minutes (queued for 2 seconds)
Status Job ID Name Coverage
  Build Pkgsrc
passed #12019
build_pkgsrc:stretch

00:00:38

 
  Build Pkg
passed #12020
amd64
build_pkg:stretch:amd64

00:01:39

passed #12021
arm64
build_pkg:stretch:arm64

00:04:21