Fix GitLab project import when a user has access only to their default namespace

## What does this MR do?

It fixes a bug when a namespace ID was not passed to `/import/gitlab_project/new` page. It occurred when a user have no choice of the namespace, so we did not render the input for namespace ID. This MR introduces a hidden input for the described case.

## Does this MR meet the acceptance criteria?

- 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 it does - 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 #23507

See merge request !6995