Scrub personal user info when disabling a user

So from the meeting in Oxford and #39 we would reserve the right to reverse it should we desire to, e.g. user says my account was hacked and everything is deleted. Not saying that we'd do this but the idea was to remove the personal information (email), change the display name and then hash the login name so we might be able to re-enable the account. 

I thought that if the login is unique then the hashed login would not fail the [unique login index criteria](https://github.com/zooniverse/Panoptes/blob/master/db/schema.rb#L216). Perhaps I should add a retry block with a suffix appender?