Allow user to import project using a different project name
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### Problem
#### Initial problem
When a user tries to import a project using a Path that has already been taken, the UI shows an unhelpful error and a 422 error is seen on the console.

```
Processing by Import::GithubController#create as JSON
Parameters: {"repo_id"=>"16008924", "target_namespace"=>"tauriedavis", "new_name"=>"portfolio", "ci_cd_only"=>false, "github"=>{"repo_id"=>"16008924", "target_namespace"=>"tauriedavis", "new_name"=>"portfolio", "ci_cd_only"=>fals
e}}
Unable to save project. Error: Name has already been taken, Path has already been taken
Completed 422 Unprocessable Entity in 323ms (Views: 0.3ms | ActiveRecord: 43.3ms | Elasticsearch: 0.0ms)
```
#### Initial fix
We updated the error message to give the user more information in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/19467.

### Proposal
We should improve the UX further by providing the user with a way to import the project with a new path name.
issue