Adding Dealer Reg confirmation email

Fixes #430. Also creates an email for #434, but there's nothing that sends this email yet.
We probably shouldn't say "Marketplace table" since that implies they only have one table.  We should probably say

```
Your payment for {{ group.tables }} Marketplace table{{ group.table|pluralize }} ...
```

Otherwise we might have people emailing asking whether their preregistration for multiple tables has gone through.