Ability to disable autoclosing issues on MR acceptance

Some projects have an intermediate step after acceptance where the issue is reviewed before the issue is considered as "closed".

There are multiple ways this could be handled:

• add referenced issue bits to the MR request (so that dealing with this case be automated outside gitlab) (most flexible and doesn't require any configuration bits);
• add an option to the project to disable this behavior (simplest?); or
• add an option to the project to assign labels to issues that would be closed rather than closing them (more complicated).