Dealer reg confirmation fix

Somehow the last PR was missing the bit of code that actually sends emails.

This PR also includes suggested text changes
This is actually not the preferred way to do this - the idiomatic way would be to put a new ``MarketplaceEmail`` in ``automated_emails.py``

The reason why we have a call to ``send_email`` here in the first place is that this is an automated email which does NOT go to the group being registered, but rather goes directly to the ``MARKETPLACE_EMAIL``.  But for all emails that go directly to attendees and groups, it's far preferable to define a ``MarketplaceEmail`` in ``automated_emails.py``

The main reason is that the way we have it here, this email will only be sent if someone preregisters with the Dealer application form.  But there's also an "Add Dealer" workflow in the admin side which Danielle uses a lot, especially when she's registering people on-site at Otakon.  So this wouldn't send the email to those people in that case.