Enable Users API to show an individual user by username

Description

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/43842

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.

Proposal

Allow the GET /users/:id/ endpoint to accept a username as well as an ID

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