GitLab

Less intrusive system note

## What does this MR do?
This MR is for making system notes less intrusive by implementing proposed design
 
## Are there points in the code the reviewer needs to double check?
System notes are reusing in issues, commits and MR discussions. May need to double check to make sure that the design is not breaking anywhere

## Why was this MR needed?
This merge request solve issue #19797 

## Screenshots (if relevant)
**Before:**
![merge-request-before](/uploads/91953598087a613078d80333a43815a7/merge-request-before.png)
**After**
![2016-11-15_17.41.46](/uploads/c809ba53cc34ee83bb439ed741dc39f3/2016-11-15_17.41.46.gif)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?
#19797, #23129

See merge request !6755