Check if an invite has already been sent to email

Loomio doesn't appear to check if an invite has already been sent to a particular email address. Be good if it did so that duplicate invites were not sent out.

If you invite a load of emails and the system spots some that have already be invited, if should send out all the others and then ask what to do about the rest, i.e. a message would say something like 'these emails/ people have already been invited, but you can send a reminder if you like" and then present a list of the already invited people/ emails with check boxes next to and a "send reminder" button.

See also these threads related to invites:
https://www.loomio.org/discussions/5130
https://www.loomio.org/discussions/3283

Issue created here: https://github.com/loomio/loomio/issues/915