TPAC: Add physical_dept arg/cookie

TPAC: Add physical_dept arg/cookie

Add physical_dept variable to TPAC -- set by url argument, backed by
a session cookie.

This is accessible to TPAC templates as ctx.physical_dept, and can
be used by conditionals to show/hide UI components based on the
provided location of a catalog terminal within a library.

physical_dept is independent from, and can be used in combination
with, physical_loc: /eg/opac/home?physical_loc=2&physical_dept=teen

Signed-off-by: Jeff Godin <jgodin@tadl.org>