Invitations API, resend

Further to #683 , if someone doesn't respond to the original invitation email (whether sent out manually or via the API, though it's the API that I'm more interested in), a further attempt gets an error back from the API. The problem is that people constantly don't respond to invitations (despite asking for them) and/or lose the emails. We'd very much like to be able to send a further group invitation even if someone has already been sent one, and irrespective of whether they are already registered in Cyclescape (typically they won't have a password, though; but not necessarily - they may have signed up themselves and requested group membership within cyclescape, but failed to act on it previously).

Start by reading issue #683 and tracing the invitations API path for group invitations. Check how prior invitations and existing Cyclescape accounts are handled; done means a further group invitation can be sent through the API whether or not an earlier invitation exists or the recipient is registered.