#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

ema/templates/ema/detail/view.html

@@ -73,11 +73,11 @@
                     <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>
-                            {% for user in obj.users.all %}
+                            {% for user in obj.user.all %}
                                 {% include 'user/includes/contact_modal_button.html' %}
                             {% endfor %}
                         </td>