Phase 2 - Replace old events and contributions tables

#95 (closed) introduced rebuilt_events and rebuilt_contributions tables. We should use the new tables whenever this data is needed on Rails app.

1. Add a feature flag on all places where fetch events and contributions tables to get data from rebuilt tables, keep in mind we need to take organization_id into account when using the new tables.
2. Populate rebuilt_contributions and rebuilt_events with old data.
3. Rollout the feature flag
4. Remove old events and contributions tables