#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

konova/admin.py

@@ -98,6 +98,18 @@ class DeadlineAdmin(admin.ModelAdmin):
     ]
 
 
+class DeletableObjectMixinAdmin(admin.ModelAdmin):
+    class Meta:
+        abstract = True
+
+    def restore_deleted_data(self, request, queryset):
+        queryset = queryset.filter(
+            deleted__isnull=False
+        )
+        for entry in queryset:
+            entry.deleted.delete()
+
+
 class BaseResourceAdmin(admin.ModelAdmin):
     fields = [
         "created",
@@ -109,7 +121,7 @@ class BaseResourceAdmin(admin.ModelAdmin):
     ]
 
 
-class BaseObjectAdmin(BaseResourceAdmin):
+class BaseObjectAdmin(BaseResourceAdmin, DeletableObjectMixinAdmin):
     search_fields = [
         "identifier",
         "title",
@@ -126,13 +138,6 @@ class BaseObjectAdmin(BaseResourceAdmin):
             "deleted",
         ]
 
-    def restore_deleted_data(self, request, queryset):
-        queryset = queryset.filter(
-            deleted__isnull=False
-        )
-        for entry in queryset:
-            entry.deleted.delete()
-
 
 
 # Outcommented for a cleaner admin backend on production