[db] add fields 'sold' and 'price_reduced' to ab4s_booking table
[booking-edit] add checkbox for agents to mark booking as sold and price reduced in booking edit in members area
[model] add isAgent() method for PluginAb4sUser
[booking-edit] refactor condition check for agent for sold & price_reduced
[artwork] add images for sold & price reduced flags
[booking-list] add sold & price reduced flags to booking thumbnails
[booking-details] display sold & price reduced flags over image slide
[booking-list] rename css selector pricereduced-flag* to priceReduced-flag*
[booking-flag] rename CSS selector #newBooking-flag to #newBooking-flag-thumbnail AND change flag image
[booking-flag] add flag for new booking in booking details