fix resending messaging for invitations

fixes #CNVS-5002
fixes #CNVS-14938

the route this posted to needed
to be made "optional" for the final
param (:id) to accept posts correctly.

Also the "pending" check on invites
requires the currentRole to be passed to it.

Test Plan:
 - Login as a teacher
 - Invite a new student
 - try to resend the invitation
 - you *SHOULD NOT* be told they've accepted it already
 - also sending the invite should not fail

Change-Id: I005699f70f0e6dadaa47adf797d6fa1a5002b1bd
Reviewed-on: https://gerrit.instructure.com/39753
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ryan Shaw <ryan@instructure.com>
Reviewed-by: Jason Madsen <jmadsen@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Ethan Vizitei <evizitei@instructure.com>