Issue #4270: Recursive option for files through API

## What does this MR do?
- Adds recursive param to tree API request. With this param we can get all repository paths in a single request. 
- Related [old github pull request](https://github.com/gitlabhq/gitlabhq/pull/9311)

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

## Why was this MR needed?
Requested in #4270 
## Screenshots (if relevant)

## 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)

## What are the relevant issue numbers?
Requested in #4270

See merge request !6088