Fix "Unsubscribe" link in notification emails that is triggered by anti-virus

## What does this MR do?

* The unsubscribe link in an email body only unsubscribes automatically when logged in, otherwise the user is asked for a confirmation.
* The unsubscribe link in an email header unsubscribes automatically whether logged in or not.

## Are there points in the code the reviewer needs to double check?

This addresses all the comments from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5241, I think!

## Why was this MR needed?

People were getting unsubscribed automatically by AV software.

## Screenshot

![Screen_Shot_2016-09-20_at_09.51.30](/uploads/083ee2865f1ad6c08e2ed97f1c4e7d0d/Screen_Shot_2016-09-20_at_09.51.30.png)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [x] Added for this feature/bug
  - [ ] 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 you do - 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?

Fixes #17198.

See merge request !6223