* adds geometry validity checks for SimpleGeomForm is_valid()
* shows validity problems on the form if a feature is invalid
* optimizes merging of different features into one MultiPolygon
* further enhances tests
* adds as_feature_collection() method on Geometry model for converting geom MultiPolygon attribute into FeatureCollection json holding each polygon as an own feature -> makes each polygon selectable in new netgis map client
* adds geom back writing to form field in case of invalid geometry, so the invalid geometry will be shown again
* updates tests
* fixes bug where race condition of celery workers could lead to duplicates in parcels (needs migration)
* adds tests for team sharing
* extends the API for team sharing support
* adds shared_teams property shortcut for ShareableObjectMixin
* adds full support for team-based sharing to all views and functions
* simplifies ShareModalForm
* adds/updates translations
* adds team sharing field to share form
* splits sharing logic into user based and teams based
* adds TeamAdmin for admin backend
* adds validity check on Team name -> only unused names shall be valid
* adds tests for compensations (WIP)
* refactors some dummy data generating into base test class
* fixes bugs detected by testing
* adds important requirements.txt change for itsdangerous package (<1.0.0 for compatibility to django-simple-sso)
