Expose User's ID in the `/admin/users/` show page

Description

Various operations on individual users via the API requires that the user's ID be specified. Right now, it's not easy to determine the ID for a user as there are no endpoints to retrieve a user's info by username.

If an admin wants to show, edit or delete a user they would need to gather the ID from a list.

If GitLab exposes the user's identifier in the web interface it would be made easier.

Proposal

Add a field to display the user's ID in the User's show page in the Admin Dashboard

Links / references

Users API endpoints requiring an ID:

• SHOW - https://docs.gitlab.com/ce/api/users.html#for-user
• UPDATE - https://docs.gitlab.com/ce/api/users.html#user-modification
• DELETE - https://docs.gitlab.com/ce/api/users.html#user-deletion