Allow users to select which letter will be used in their Collaborative editing text cursor

Issue Summary

As of today, users with the same first name will be assigned the same letter to their Collaborative text cursor.

If we take in consideration the behaviour we have under CONFSERVER-59400, the usability of our collaborative editing feature is considerably degraded

Steps to Reproduce

1. Create a new page
2. Edit it with multiple users (be sure to have at least two of them with the same first name)

Expected Results

We are able to discern between them easily

Actual Results

Users will be assigned the same initial (first name initial), making it hard to discern between them.
If we take in consideration the behavior under CONFSERVER-59400, it becomes impossible to discern who is editing where.

Example being:

Workaround

Currently there is no known workaround for this behavior. A workaround will be added here when available