Updated registration email templates.

* Changes to helpers probably wasn't necessary. I was looking at it to find out what templates were used by the send_email function. I do think the change is clearer, though.
* for the password reset email, we could probably just pass in site as a context variable, but I did it this way so that it was explicit what I was after. Once we decide which context to pass in, we should be ok.
We can mark the file as unicode, but I don't think we really do that anywhere else.