Enable editing of email field in quick-create user

From Derek:

>Can we enable editing the email field when creating new users in Reservations?

![image](https://cloud.githubusercontent.com/assets/6618711/3293092/fdc6f970-f596-11e3-890f-42d31310bae8.png)

>Summer students don't have a Yale email address so we need to be able to enter their personal email address in this field. Example in this case is netid=sc2262. 

I told him staff and admins can create users via users->new user but there's not really a reason to disallow editing of these fields, especially since users can go and edit their name, email, etc. after their account is created anyway (though maybe this will change in #528). At the very least though, their email should be editable.