Add multiple user id's to a team

As a user, I can invite multiple user id's to my team at once. Ideally, permit team moderators to input a comma-separated list into the "add member" form to enable multiple add/sinvites.

Start at the team “add member” form and trace the existing single-user invite flow through its submission handler and invitation endpoint. Confirm how comma-separated user IDs are parsed and validated, then verify that each valid ID receives an invitation and that the existing single-user behavior remains unchanged.

Written by the indexing model from the issue text.