OO-753: remove the optimistic locking from SecurityGroupMemebership and make almost all field immutable

OO-749: check if the controller is disposed before updating the business path (with back is possible)

OO-747: in case of a login with no sites, no tabs, place a dummy panel and a gui stack to prevent the red screen

OO-705: switch the format of pdf's thumbnails to png, implements small admin panel to reset the thumbnails

OO-702: remove the fallback if there isn't any mailing definition, check first the default settings and send e-mails only if it's mandatory

OO-662: fix bad performance in catalog if instance contains a certain amount of members only courses