Expose Discord ID to API

What does this MR do and why?

Exposes the Discord ID field on users to the API (GET, PUT and POST).

Screenshots or screen recordings

How to set up and validate locally

• GET /api/v4/users/:id - you should see a discord field in the response
• PUT {"discord": "11111111111111111"} to /api/v4/users/:id
• POST {..., "discord": "11111111111111111"} to /api/v4/users

Validate - navigate to /:username and check the displayed ID matches.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #390671 (closed)