Don't auto-logout TPAC in staff client

Wrap the meta refresh in TPAC within a check for ctx.is_staff, so
that idle TPAC tabs will not log out when running within the staff
client.

This is intended to resolve the primary cause of symptoms mentioned
in bug 1036318 "OPAC timeout within the client"

Signed-off-by: Jeff Godin <jgodin@tadl.org>
Signed-off-by: Jason Stephenson <jstephenson@mvlc.org>