signon: allow searches in the admin site

Signed-off-by: Mattia Rizzolo <mattia@debian.org>

signon/admin.py

@@ -2,4 +2,9 @@ from django.contrib import admin
 from .models import Identity
 
 
-admin.site.register(Identity)
+@admin.register(Identity)
+class IdentityAdmin(admin.ModelAdmin):
+    search_fields = (
+        "username",
+        "person__fullname", "person__email",
+    )