Option to disallow author to approve merge request

Description including problem, use cases, benefits, and/or goals

To prevent having to add Approvers amount + 1, because the author can approve themselves, it is suggested to not allow the author of the MR be able to approve a MR.

Proposal

Non-configurable. Simply don't allow the author to approve.

This does not apply to Accepting merge requests, which should be tackled separately.

Links / references

Original issue

Similar to gitlab-org/gitlab-ce#5940 (Don't allow accepting MR with failed build)

Zendesk issue: https://gitlab.zendesk.com/agent/tickets/17096

Strictly do not allow merge request author to accept or approve their own merge request, optionally.

cc/ @JobV