Implement overall change tracking in OSEM

Open Source Event Manager (OSEM) is an event management application tailored to Free Software Conferences. Software conferences tend to have multiple people in charge of its organization.This can often lead to confusion as one organizer may not be aware of the changes made by another organizer/user to some aspect of the conference. The primary aim of this project is to create a page similar to a Changelog which would allow organizers to have an overview of all the changes made to all aspects of a conference and allows them to take related actions.This provides organizers with a higher degree of control over the organization of their conferences.