* adds button and functionality for leaving a team
* if the admin leaves the team, another user will be chosen as new admin automatically
* improves Team (django) admin backend
* better control over user adding-removing
* only added team members are selectable as admin
* 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
* 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
* refactors: attached files can only be downloaded by users with shared access
* adds hint on unshared data being unshared for compensation, ema, eco-account
* minor optical enhancement for user contact form
* adds user/includes/contact_modal_button.html for calling contact data of a user
* adds user contact modal to intervention detail view
* adds translations
