Add users search results to global search

What does this MR do?

Adds a users search result tab to the global search.

• Global context search
• Group context search
• Project context search

The development of this MR is sponsored by @ siemens (/cc @bufferoverflow).

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/25973

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guidesis