Allow staff to pick search/pref libraries

Allow staff to pick search/pref libraries

This adds a workstation-level set of settings for search and preferred
libraries. These override the default search library and the preferred
library individually, allowing staff to default searching at one OU but
still get copy information for a different one.

Original use case is "search everywhere, but show my local copies".

Signed-off-by: Thomas Berezansky <tsbere@mvlc.org>