Allowing changing Guest name

[sysrqb]

Which part of the project should be enhanced? Guest/anonymous sessions are assigned a Guest name, however this name is not editable (afaics).

Is your enhancement request related to a problem? Please describe.

Following #611 and hedgedoc/hedgedoc#505, allowing modification of the guest name will let users clarify who they are (when appropriate).

Describe the solution you'd like

Allow changing the Guest name, as it is shown in the "Online" drop-down list and as it is associated with authorship attribution.

Additional context

This is a feature Etherpad provides, and it is very valuable.