New activities system
requested to merge feature/activities into 39-split-activities-into-tasks-observations-and-activities
Continues #39 (closed) with the new activity system:
- the old "activities" table is now the "tasks" table and model
- a new "activities" table was created with migration that fills it from existing data
- Activities are accessible via the "activity" button in garden menu
- Map models now triggers new activities during their lifecycle