Add search field for identities

Makes it possible to search for identity values in admin gui.

greg/admin.py

@@ -89,6 +89,7 @@ class RoleTypeAdmin(VersionAdmin):
 class IdentityAdmin(VersionAdmin):
     list_display = ("id", "person", "type", "verified")
     list_filter = ("verified",)
+    search_fields = ("value",)
     readonly_fields = ("id", "created", "updated")