Fix unverified users not being able to sign in

Even if an email address is marked unverified, a user should still be able to sign in with the address. Once the user signs in, they will be forced to verify their ownership of the address.
Add tests to ensure password reset reverification works as expected from both the auth screen and the email picker.
Merge pull request #3821 from mozilla/issue-3811-unverified-signin

Issue 3811 unverified signin