Feature: Author invitation

The issue addressed - #634

Feature includes:

1. Sending an author invitation email from the control panel
2. Once an author accepts the invitation, he/she is added to the manuscript team, and team members
3. An author can submit and revise the manuscript he/she accepted an invitation for
4. If invitation status is declined, the user is shown a successful declination page
5. Once accepted/declined, an invitation cannot be used again