Fix #701 Preferable default theme (#746)

* Make it possible to update users.preferred_theme to NULL

* Make preferred them null when 'Default theme' is selected

* Run cargo fmt

plume-models/src/users.rs

@@ -54,6 +54,7 @@ pub enum Role {
 }
 
 #[derive(Queryable, Identifiable, Clone, Debug, AsChangeset)]
+#[changeset_options(treat_none_as_null = "true")]
 pub struct User {
     pub id: i32,
     pub username: String,