Soft-deleted user accounts should allow duplicate emails

[issa-tseng]

We are soft-deleting accounts and formdata, so that recovery is possible. But this means that if an account is deleted, a new one cannot be created at the same address, as the unique constraint still applies.

The likely solution is to bind the unique constraint on a compound (email, deletedAt). But because account deletion isn't actually in the initial release, we can punt.

[issa-tseng]

probably prioritize for 0.5.0 if we are introducing account deletion there.