#146 Team leave

* 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

user/templates/user/team/index.html

@@ -46,6 +46,9 @@
                             {% endfor %}
                         </td>
                         <td>
+                            <button class="btn rlp-r btn-modal" data-form-url="{% url 'user:team-leave' team.id %}" title="{% trans 'Leave team' %}">
+                                {% fa5_icon 'sign-out-alt' %}
+                            </button>
                             {% if team.admin == user %}
                                 <button class="btn rlp-r btn-modal" data-form-url="{% url 'user:team-edit' team.id %}" title="{% trans 'Edit team' %}">
                                     {% fa5_icon 'edit' %}