#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/forms.py

@@ -317,6 +317,15 @@ class RemoveTeamModalForm(RemoveModalForm):
     pass
 
 
+class LeaveTeamModalForm(RemoveModalForm):
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        self.form_title = _("Leave team")
+
+    def save(self):
+        self.instance.remove_user(self.user)
+
+
 class TeamDataForm(BaseModalForm):
     name = forms.CharField(
         label_suffix="",