minor UI tweaks; introduced hook system for miscellaneous customizations.

minor UI tweaks; introduced hook system for miscellaneous customizations.

The hook system is experimental; as the name suggests, it's a
mechanism for declaring and implementing hooks into various parts of
the application. Currently, I've just added one hook for determining
which users should be entitled to create new ReadingLists. We'll see
how it plays out.

git-svn-id: svn://svn.open-ils.org/ILS-Contrib/servres/trunk@906 6d9bc8c9-1ec2-4278-b937-99fde70a366f