Source user avatar initials from Jupyter Server

[markgrahamdawson]

Problem

The workflow toolbar avatar is currently getting the users first initial as the first value in the username string.
this.user.username[0].toUpperCase()
This is ok, but sometimes this name is an unexpected value, resulting in a confusing letter being presented in the avatar.

Proposed Solution

An alternative option would be to get the users initials from Jupyter Server.

The initials field may or may not be provided so we need to be robust to its absence (using the first letter of the username is a good option if initials aren't provided).