Confidential merge request doesn't contain helpful details
### Problem to solve
Confidential merge request doesn't have a lot of details when created.
### Further details
When a confidential merge request is created it has a title based on the branch name and no description. It's not that helpful.
### Proposal
From @rspeicher's comment on https://gitlab.com/gitlab-org/gitlab-ce/issues/58583#note_189521675:
> 1. It'd be nice if we filled in the MR description with a link back to the original confidential issue, at the very least. I'd also prefer if the MR title reflected the issue title, though I understand that is itself a potential security issue.
### What does success look like, and how can we measure that?
Confidential merge request will be created with title based on the confidential issue's title and description will has a link back to confidential issue.
### Links / references
/cc @jramsay @rspeicher @iamphill
issue