Move Labels and Milestones as sub tab to Issues/MR

## What does this MR do?

UI/UX change. Moves project labels and milestone tabs under Issues and Merge request tabs. 

## Why was this MR needed?

To solve problem of having to many tabs in top navigation. Since Labels and Milestones depends on Issue/MR feature it belongs to it and can be removed from top nav.

## What are the relevant issue numbers?

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/18368

## 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) - not needed
- [x] API support added - not needed
- [x] Tests
  - [x] Added for this feature/bug - changed existing tests to match new behaviour
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)

cc @jschatz1 @annabeldunstone @skyruler

See merge request !4592