* 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 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
