Display full name (#20171) (#20246)

Display full name (#20171) (#20246)

Backport #20171

The setting `DEFAULT_SHOW_FULL_NAME` promises to use the user's full name everywhere it can be used.

Unfortunately the function `*user_model.User.ShortName()` currently uses the `.Name` instead - but this should also use the `.FullName()`.

Therefore we should make `*user_model.User.ShortName()` base its pre-shortened name on the `.FullName()` function.

Co-authored-by: Baekjun Kim <36013575+kimbj95@users.noreply.github.com>