Adapt the `Community-Statistik+KPI-Uebersicht.ipynb` to the new database schema

[hugotiburtino]

We've been simplifying the database in the last time.
See the following issues with the corresponding PRs

• Merge entity_revision_field into entity_revision db-migrations#348 -> feat: Merge entity_revision_field into entity_revision db-migrations#349
• Simplification of the taxonomy_ tables db-migrations#346 -> feat: Simplify taxonomy tables db-migrations#375
• Major simplification of event_* tables db-migrations#344 -> feat: Simplify event tables db-migrations#374

They will be deployed soon in serlo.org. So we need to adapt the Community-Statistik+KPI-Uebersicht.ipynb so that it continues to work after these changes.