* 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
