Prevent excpetion at user subscription when already is subscribed to a conference and add hanling to the similar unsubscribe event.
remove double validation at app/models/subscription.rb
update subscription_controller_spec
Sets up paper_trail tracking in important models
and add conference_id to versions for storing conference_id
of conference related objects as metadata.This will help in querying
for versions related to conferences.
Fix issues with factories & event types initialization.Import paper_trail testing helpers
Fix bug: Creating an event creates a useless version with event update
Add revert and view changes features to revision history
