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](c8b2b3f7c3 (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
c003fc5091