* 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 support for DELETE method for all relevant objects
* improves get_obj_from_db functionality
* drops custom compensation logic for get_obj_from_db due to improvement of base method
* writes test for sharing using the API
* fixes bug on frontend form where an exception occured on generating a new API token if no token existed, yet
* adds permission constraint (default group) for using the api in general
* fixes default-group-only behaviour for sharing-API, so users can only add new users and not removing them, as long as they do not have any other group membership like registration or conservation office
* changes 'ksptoken' to 'Ksptoken' to match CGI standard for http header keys
* adds token checking to AbstractModelAPIView
* adds user accessibility filtering for intervention API v1
* extends fetch_and_serialize() method to take a dict for db filtering instead of a single field and value
* organizes urlnames into supporting formats like "api:v1:intervention"
* adds new app to project
* adds relation between User model and new APIUserToken model
* adds first implementation for GET of intervention
* adds basic code layout for future extension by having new versions
