Pass user instance to Labels::FindOrCreateService or skip_authorization: true

## What does this MR do?

It fixes a bug described in #23694 when `project.owner` was passed to `Labels::FindOrCreateService`. `Labels::FindOrCreateService` expected a user instance and `project.owner` may return a group as well. This MR makes sure that we either pass a user instance or `skip_authorization: true`.

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

- places where we pass `skip_authorization: true`

## Does this MR meet the acceptance criteria?

- Tests
  - [x] Added for this feature/bug
  - [ ] All builds are passing
- [ ] 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 #23694

See merge request !7093