Move user#show and group#show routing under root url

## What does this MR do?

Changes routing so user#show and group#show routing is under root url 

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

no

## Why was this MR needed?

For nice and consistent routing  for user/group show pages:

| before | after| 
|---|---|
| `/u/dzaporozhets` | `/dzaporozhets`| 
| `/dzaporozhets/gitlab-ce` |`/dzaporozhets/gitlab-ce`| 
| `/u/dzaporozhets/snippets` |`/u/dzaporozhets/snippets`| 
| `/groups/gitlab-org`|  `/gitlab-org`| 
| `/gitlab-org/gitlab-ce`|  `/gitlab-org/gitlab-ce`| 
| `/groups/gitlab-org/issues `|  `/groups/gitlab-org/issues `| 

So show page for user and group shown as `/:name` which is much nicer than before. At same time all nested pages like group issues stays under REST `/groups/:name/issues` so we don't need to introduce extra name restrictions ( ex. case when group has project with name "issues")

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] ~~[Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)~~
- [ ] ~~API support added~~
- Tests
  - [x] Added for this feature/bug
  - [x] 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 https://gitlab.com/gitlab-org/gitlab-ce/issues/3620

See merge request !6686