Disallow reopening of a locked issue or merge request

• Because this is what I observe community members doing.
• We've decided that an issue is resolved, and we close it. The community member disagrees and just re-opens it.
• Once an issue is locked, they should not be able to do that. (At least the conversation can cease, but better that we lock down closing of it too.)
• Update : Nobody can open/close a locked issue or merge request, per https://gitlab.com/gitlab-org/gitlab-ce/issues/39665#note_95577568.