Enable approvals to be allowed on project without being required per merge request

Description

Currently, if approvals are enabled for a project, the minimum number of approvers cannot be less than one (1). While this may seem intuitive there are cases where project owners may want to allow approvals but not require them. Project owners should be able to turn on approvals for a project and allow the Masters to determine if a Merge Request requires extra approvals.

Proposal

Allow Project Owners to enable approvals but allow the minimum number of approvers to be zero (0).

Links / references

Releated to #3282 (closed)

Use cases

On a particular project, I have a set of Masters that are allowed to merge. For most instances they do not require external approval for completing a merge, however, if the nature of the request is something they might not be as familiar with, I trust that they will seek the necessary guidance. This is a case where the Master should be able to add an approver and wait for approval to merge.

Feature checklist

Make sure these are completed before closing the issue, with a link to the relevant commit.