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 It's not clear which behaviour devise_invitable should have, but it is clear that we depend on it's old behaviour, so let's reinstate it.