Bug 6298 : Create new ShowReviewerPhoto preference

Bug 6298 : Create new ShowReviewerPhoto preference

This new system preference (in OPAC Features) allows libraries to
turn off reviewer photos entirely if they want to. The default is
for these photos to be shown.

Note that this setting is linked to ShowReviewer in that both of
them need to be turned ON for the avatars to be displayed.

Signed-off-by: Francois Marier <francois@debian.org>
Signed-off-by: Nicole C. Engard <nengard@bywatersolutions.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>