Allows invite modal to re-invite single users

What does this MR do and why?

• Allows users who are already invited to be invited again. See https://gitlab.com/gitlab-org/gitlab/-/issues/247208#note_835657520
• skips tasks to be done creation if it already exists for a member.

How to set up and validate locally

1. Follow these steps to reproduce this locally https://gitlab.com/gitlab-org/gitlab/-/issues/247208#note_835103747 on master branch.
2. Repeat those steps on this branch.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #247208