Merge branch 'issue_18398' into 'master'

Check for conflict with wiki projects when creating a new project.

## What does this MR do?
Check for conflict with wiki projects when creating a new project
## Are there points in the code the reviewer needs to double check?
No
## Why was this MR needed?
To avoid exposing the information from the wiki repository of other project
## What are the relevant issue numbers?
#18398 
## Screenshots (if relevant)
![Screen_Shot_2016-06-24_at_6.03.49_PM](/uploads/7bf55e5159bf0c2b653b8f4f941f72fc/Screen_Shot_2016-06-24_at_6.03.49_PM.png)
## 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
- [ ] 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)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !4918