Allow users to modify user data via API

Description

At the moment only admins can modify a user data via the PUT /users/:id. Authenticated users cannot modify their own data via PUT /user. Maybe I am missing a good reason for why this isn't implemented.

Proposal

Allow users to modify user data via PUT /user.

Limitations

The administrator might have disabled altering some information (e.g. username or email address). So only information that can be modified on the /profile page should be allowed to be changed through the API.

Links / references

https://docs.gitlab.com/ce/api/users.html#user-modification