Use permissions to authenticate bds buro members

I prefer using a permission (namely `bds.is_team`) to determine if a
user is member of the BDS staff rather that using a `is_buro` boolean
field.

We already use this approach is the kfet app

bds/migrations/0003_staff_permission.py

@@ -0,0 +1,22 @@
+# Generated by Django 2.2.8 on 2019-12-20 22:48
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("bds", "0002_bds_group"),
+    ]
+
+    operations = [
+        migrations.AlterModelOptions(
+            name="bdsprofile",
+            options={
+                "permissions": (("is_team", "est membre du burô"),),
+                "verbose_name": "Profil BDS",
+                "verbose_name_plural": "Profils BDS",
+            },
+        ),
+        migrations.RemoveField(model_name="bdsprofile", name="is_buro",),
+    ]