Broken link in email registration

`process.env.SITE_URL` is undefined in when running in development, resulting in an email registration message with this body:

```
Content-Type: text/plain
Content-Transfer-Encoding: quoted-printable

undefined/accounts/verify/$2a$10$f5tR7bsjRs2AEcVeTTDv7u9NLtJwAXGPX.luv2wkfw=
```