Expose ChatName objects to slash commands

What does this MR do?

This MR exposes ChatName objects to slash commands, instead of only exposing User objects. This is necessary for GitLab Chatops (https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4466) but can also be useful for other (future) slash commands.

EE MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4693

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

• https://gitlab.com/gitlab-org/gitlab-ee/issues/4856
• https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4466