Change PK on EG, EI db tables to include parent ids

[alasdairwilson]

so at the moment each EG and EI has a unqiue id, i.e. they cant be repeated across events.

It may be preferable top use primary keys that are combination of event.id and evengroup.id for eventgroups and event.id, eventgroup.id and eventitem.id for items. as mentioned in #144

This is a non trivial change though and doesnt aid functionalityt but will stop future urls being /436/6742 and instead be /436/1