Add setting to only allow merge requests to be merged when all discussions are resolved

Extension of the feature proposed in https://gitlab.com/gitlab-org/gitlab-ce/issues/10325 and implemented in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5022

Similar to the “Only allow merge requests to be merged if the build succeeds” checkbox in settings we already have. It would allow blocking a merge request from being merged accidentally, which people are also requesting here: https://gitlab.com/gitlab-org/gitlab-ee/issues/761