CORS fixes

* The Safari extension apparently now uses the origin `file://` and expects
  that to be returned (see bitwarden/browser#1311, bitwarden/server#800).

* The `Access-Control-Allow-Origin` header was reflecting the value of the
  `Origin` header without checking whether the origin was actually allowed.
  This effectively allows any origin to interact with the server, which
  defeats the purpose of CORS.