Fix `User#to_reference`

## What does this MR do?

Fix the method signature of `User#to_reference` so that moving an issue with a user reference does not throw a "invalid number of arguments" exception.

## Why was this MR needed?

1. Changes in 8.13 require `Referable`s that don't have a project
   reference to accept two arguments - `from_project` and
   `target_project`.

2. `User#to_reference` was not changed to accept the
   `target_project` (even though it is not used). Moving an issue
   containing a user reference would throw a "invalid number of
   arguments" exception.

3. The regression was introduced in [c8b2b3f7](https://gitlab.com/gitlab-org/gitlab-ce/commit/c8b2b3f7c32db873f1bebce3e3b1847ea24d235f#91fabb7ad88bd2fde6fef1c100a719c00e503047_75_79), which expects
   all `Referable`s that don't respond to `:project` to have a `to_reference`
   method that takes two arguments.   

## Does this MR meet the acceptance criteria?

- Tests
  - [x] Added for this feature/bug
  - [ ] 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?

- Closes #23662

See merge request !7088