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
Change in error message of validation cause previously it was:
"Price currency Currency is different.."
`paid?` method returned true even when `ticket_purchases` is empty for
a given user.
