Return to invite new user; invite multiple users in one go

Closes #3533, Closes #3547, Closes #3531
- invite-new-user always resets role to author now, allowing for multiple invites on one load
- Added confirm action when hitting return in INUModal's email input
- Don't pass users as the model to invite-new-user-modal
- Move invite-new-user reset to a `finally` clause
- invite-new-user always closes on a confirm
Merge pull request #3567 from novaugust/issue#3564

Return to invite new user; invite multiple users in one go