Generate slugs for all users

We need to generate a slug for all users in the database (and all new users), we are going to use it to create links in the FE for their profile pages.

- This task requires to update the user schema and add a new `slug` field, it should be unique and indexed in the database.
- When a new user is created we need to generate a new slug for the new user
- Update all user endpoints to find the user by `slug` or `_id`
- Create a script to assign a slug to the current user in the database