Fix sender email for event registration confirmation.

#4877 

This PR formats the sender address for an event registration confirmation to be the same as the reminder emails. See https://github.com/openscholar/openscholar/issues/4877#issuecomment-34678127.

Message details for the confirmation email are:
![selection_182](https://f.cloud.github.com/assets/4497748/2136850/f4ab5046-9319-11e3-8210-49c3fe8156b5.png)

So the sender is admin@example.com but the sender is jfk@whitehouse.gov