pub fn force_keep(&self, url: &PathMaybeWithLocale) {
        let mut order = self.order.borrow_mut();
        // Get rid of any previous mentions of this page in the order list (which will
        // prevent this page from ever being evicted)
        order.retain(|stored_url| stored_url != url);
        let mut keep_list = self.keep_list.borrow_mut();
        keep_list.push(url.clone());
    }
Forces the store to keep a certain page. This will prevent it from being
evicted from the store, regardless of how many other pages are
entered after it.

Warning: in the *vast* majority of cases, your use-case for this will be
far better served by the global state system! (If you use this with
mutable state, you are quite likely to shoot yourself in the foot.)
packages/perseus/src/state/state_store.rs