Minor Feature : username when cataloguing

Minor Feature : username when cataloguing

biblios can now be added librarian name automatically when cataloguing.
add "user" in the Default value of the biblio

[RM doc note: in other words, if in the MARC framework you set
 the default value of a subfield to 'user', the username of the
 cataloger will be entered into that subfield whenever you create
 or modify that bib in the Koha bib editor.]

Signed-off-by: Galen Charlton <galen.charlton@liblime.com>