Raise exception if more than user share email and password

We need to handle a race condition where when a user quickly registers
twice. The RegisterUserMixin already deals with it, but expects a
MultipleObjectsReturned exception. This is now correctly raised.

Fixes #1313
Rename Emailbackend to EmailBackend

The non-camelcased class has bothered me for a long time.
We can remove the deprecated spelling for version 1.0 :)
Management command to find duplicate email addresses

Oscar currently treats the local part of an email address in a
case-sensitive way. That's standards compliant, but the majority of
users is not aware of it and most SMTP server setups ignore casing.

In preparation for switching to a case-insensitive handling of email
addresses, this commit adds a management command to find duplicate email
addressses. Casing is ignored.

Related to #1179.
Disable instead of delete sers with duplicate email addresses

If an external system introduced duplicates, we really shouldn't just
blindly delete the users. This change logs the event and instead
disables them.
Some more redirect() instead of HttpResponseRedirect(reverse())

The file raised a PEP8 line length warning, so I did a bit of
refactoring along the way.