Enforce unique emails on User

[pbrisbin]

Caveats:

• This is only at app-level, no DB constraint (yet)
• This is only on new users, not profile updates to existing users (yet)

Rationale:

Adding the DB constraint fails almost all of our tests. Our user factory actually does create users with unique emails, but authenticating with the dummy plugin always attempts to update it to the same address. Modifying that behavior will be both messy and unrelated to this validation, so I want to do it separately.

I thought it made sense to do this first, since it should prevent creating users with non-unique emails in production while I'm working on adding the more robust, DB-level constraint.

Once the constraint is in and the test behavior corrected, I'll change the update path to also handle uniqueness violations gracefully.

[pbrisbin]

Adjusting the tests to allow for unique emails was easier than I thought. #226 should be merged first, then I'll rebase this (and add code for the update path).

[jferris]

226 should be merged first, then I'll rebase this (and add code for the update path).

This is #226?

[pbrisbin]

Sorry, #227, where we're having the other discussion.

[pbrisbin]

Just realized: once the merge-on-existing-email logic is in place, this path would never be hit. Closing in favor of that, which I'll open soon.