Only show register tab if signup enabled.

## What does this MR do?

Fixes a regression where the register tab is activated, even if sign-up enabled is not activated in application_settings.

## Screenshots (if relevant)

When signup is disabled: 

![Screen_Shot_2016-10-22_at_8.56.23_PM](/uploads/bd0fa5f27114779c0d290a8151c1c253/Screen_Shot_2016-10-22_at_8.56.23_PM.png)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG.md) entry added
- [ ] 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?

https://gitlab.com/gitlab-org/gitlab-ce/issues/23654

See merge request !7058