Fix confusing description of a blocked user.

## What does this MR do?
Rewrite confusing description of a blocked user in admin settings. If someone is blocked, the same message is displayed as if he is unblocked.

## Are there points in the code the reviewer needs to double check?
No.

## Why was this MR needed?
Because it seems that this is a cosmetic bug.

## What are the relevant issue numbers?
None.

## Screenshots (if relevant)
![blocked_user](/uploads/0782a82760ae19661d69c63a97daaf33/blocked_user.png)

## 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)
- [x] API support added
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [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)

See merge request !5582