allow new users to be created with same email as deleted users

@JDutil NOTE: currently this spec is red. I would expect that we'd want users to be able to create accounts with the same email that they had used previously, if for example they deleted their account some years ago and want to make a new one.

What do you suggest as a solution here? I'm thinking we either:

(1) Slightly relax the enforcement of unique user emails so that it's only for non-deleted users.

(2) Change the email address of a user when we destroy them.

Of the two, I slightly prefer (2) but can see the argument for either.

We could, for instance, change `user@example.net` to `user@example.net[deleted-2014-09-04-09-35]` or something like that. What do you think is the best approach here?