Fix: Really truncate long usernames

Regression of MediaWiki UI icon class.
Follow Up: I27638602a0c3df6af40f13823e64e0ab31cc03c9

Bug: 72828
Change-Id: Ifa292b0a9c2169cbcd0a457fa44fd5b6a2d90ad3
(cherry picked from Ie59615f8999145f0b88e868e56086b5858a03feb)