#169 Team delete-restore

* adds restorable delete functionality to Team model
* refactors minor code model parts by introducing DeletableObjectMixin
* only non-deleted Teams can be chosen for sharing
* deleted Teams can be restored using the proper function on the backend admin
* deleted Teams do not provide
* adds migration

intervention/templates/intervention/detail/view.html

@@ -125,7 +125,7 @@
                     <tr>
                         <th scope="row">{% trans 'Shared with' %}</th>
                         <td class="align-middle">
-                            {% for team in obj.teams.all %}
+                            {% for team in obj.shared_teams %}
                                 {% include 'user/includes/team_data_modal_button.html' %}
                             {% endfor %}
                             <hr>