Admin able to update DisplayName of user

[powdermouse]

Is your feature request related to a problem? Please describe

We use SSO to log in, if the user is renamed in AD, the following changes must be made (otherwise a new user will be created):
email, username, displayname

Describe the solution you'd like

It would be nice to change DisplayName from the admin interface.

Changing the display name, user name and email should also be possible via API.

Describe alternatives you've considered

I (and the user) are currently making the changes manually and before the user logs in again, which is very error prone.

[sy-records]

With support for changing username and email in #980, do we need support for changing displayname? cc @fenbox

[fenbox]

@sy-records Yes, I agree.