Reinstate password reset flow for some users

Since we upgraded to devise_invitable 1.1.5 in 64a3c14fff53cd8536233eb658e6137b05ea1a31
password reset hasn't worked for users who have been invited but haven't accepted.

This is because devise_invitable 1.1.1 changed the behaviour of invite! such
that it no longer sets confirmed_at when inviting/creating a user:
https://github.com/scambra/devise_invitable/commit/38e588195e1ade2241440c660b309ee1594ab8ab
Fix password reset for some existing users

Where "users" is users invited but not accepted, created during the window
where new users didn't have confirmed_at set at create-time.
Merge pull request #59 from alphagov/fix_password_reset_for_invited_but_not_accepted_users

Reinstate password reset flow for some users