Navigation improvements

- [x] make explore area as part of dashboard
- [x] move navigation links from top right corner to sidebar
- [x] group navigation items on dashboard
- [x] cleanup UI after such changes
- [ ] fix tests

cc @sytses @JobV

See merge request !1177

Feature/control import options

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

* projects from explore page are available from dashboard
* use dashboard sidebar for both logged in and not users
* move explroe projects under projects tab
* move explore groups under groups tab
* move public snippets under snippets tab
Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Improved 2FA warning message about lost codes

Fixes internal issue: https://dev.gitlab.org/gitlab/gitlabhq/issues/2472

The warning message now clearly states that access will be lost of the 2FA recovery codes are not saved.

/cc @sytses @JobV

See merge request !1175