Unify update_merge_request ability for projects and issuables

We have update_merge_request rule in https://gitlab.com/gitlab-org/gitlab/blob/master/app%2Fpolicies%2Fproject_policy.rb#L250 and in https://gitlab.com/gitlab-org/gitlab/blob/master/app%2Fpolicies%2Fissuable_policy.rb#L19

What about keeping only one of them and probably move it into https://gitlab.com/gitlab-org/gitlab/blob/master/app%2Fpolicies%2Fmerge_request_policy.rb?