v1.4.0: renamed to Ownlist, optional credit link

Changed

• The template is now called Ownlist and lives at
  pfstr/ownlist. GitHub redirects the old
  pfstr/newsletter-template URLs; to be explicit, point your update remote
  at the new address:
  git remote set-url template https://github.com/pfstr/ownlist.
  New deployments default to the Worker name ownlist and the D1 database
  ownlist-db. Existing deployments keep theirs: the update steps in the
  README leave your wrangler.json untouched.

Added

• Credit link: a small "Powered by Ownlist" line, linking the
  project page, under the signup and embed forms, on the confirmation and
  unsubscribe pages, and in the email footer. On by default; set
  SHOW_CREDIT to "pages" or "email" to show it only there, or to
  "false" to hide it. The link carries UTM parameters (source, medium,
  placement) and nothing else.