Optionally require email validation before account is usable.

Reworked to add new `isVerified` field and to remove validation when signing up with social accounts.

There is no way in the admin UI to change the `isVerified` status, nor is there a way for the user to request the verification to be resent. I don't know if these are required right now or not. (Resetting `isVerified` back to `no` implies resending a verification email, which might be weird. Perhaps the best solution is to only allow `isVerified` to be set to `yes`?)

I tried to update code to match the latest UI and coding style changes, but it's entirely possible I missed something.