Ensure MR approvals don't get stuck

The potential approvers of an MR are:

- The users in the approvers list (excluding other permissions, as long
  as they can see the MR).
- Users with developer access or higher on the project or the group
  containing the project.

This number is reduced by:

- The author not being able to approve their own MR.
- Users who have already approved the MR.

If the number of potential approvers is less than the number of
approvals left by the normal count, pick the number of potential
approvers instead, so the MR isn't 'stuck' in a state where it needs
approval but no-one can approve it.