UserArea: Make unique validation of emails optional

[HeyJoel]

If user area does not use email address as a username, then the email uniqueness checking should be optional. Requiring a unique email during registration allows the possibility of revealing a registered email address, which can be problematic for certain types of sites where registration should be confidential.

The default configuration should allow for duplicate email addresses (unless used as a username), which is a breaking change in behavior.

[HeyJoel]

Fixed, will be released in v0.10