Fetch user initials from UIServer

[markgrahamdawson]

Closes #1504

The userprofile endpoint has been extended in cylc/cylc-uiserver#510 so it now provides user initials.

If for whatever reason the initials field is empty the avatar will be a user symbol

And if the initials field is populated the avatar will look like one of the two

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• Tests are included (or explain why tests are not needed).
• CHANGES.md entry included if this is a change that can affect users
• Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[markgrahamdawson]

Is a changes.md entry required for this one?

[oliver-sanders]

No need for a changelog entry for this ticket (no one is going to check the cylc-ui changelog in the first place to major things only)

[oliver-sanders]

Looks good.

Note to reviewers, PAM doesn't return a helpful initials field, but other authenticators can.