Adding Dealer Reg confirmation email

Fixes #430. Also creates an email for #434, but there's nothing that sends this email yet.
We should add this at the end:

```
{{ MARKETPLACE_EMAIL_SIGNATURE }}
```

Which is what we're doing in our other Marketplace emails.  Not a big deal, but kind of nice to have a personal touch at the end, even if that personal touch was generated by a cold, unfeeling Django template :P