[30025] Split BiblioAddsAuthorities into 2 preferences

This patch splits the system preference BiblioAddsAuthorities into two new system preferences that more clearly define what the settings do:
1- RequireChoosingExistingAuthority: this preference indicates whether a cataloger must choose from existing authorities, or if they can enter free text into controlled fields
2- AutoLinkBiblios: this preference determines whether Koha will attempt to link a new record to existing authorities upon saving. In conjunction with the existing preference, AutoCreateAuthorities, unmatched headings will either be linked to a new authority, or remain uncontrolled