pm_list: Handle narrow functions separately for pm_section.

This commit is a preparatory commit which introduces
`handle_narrow_deactivated` and `handle_narrow_activated`
functions in pm_list.js, separately from top_left_corner.js,
to reduce the complexity of handling private messages section
separately.

static/js/pm_list.js

@@ -1,6 +1,7 @@
 import $ from "jquery";
 
 import * as narrow_state from "./narrow_state";
+import * as people from "./people";
 import * as pm_list_data from "./pm_list_data";
 import * as pm_list_dom from "./pm_list_dom";
 import * as stream_popover from "./stream_popover";
@@ -74,3 +75,36 @@ export function update_dom_with_unread_counts(counts) {
     update_private_messages();
     set_count(counts.private_message_count);
 }
+
+function should_expand_pm_list(filter) {
+    const op_is = filter.operands("is");
+
+    if (op_is.length >= 1 && op_is.includes("private")) {
+        return true;
+    }
+
+    const op_pm = filter.operands("pm-with");
+
+    if (op_pm.length !== 1) {
+        return false;
+    }
+
+    const emails_strings = op_pm[0];
+    const emails = emails_strings.split(",");
+
+    const has_valid_emails = people.is_valid_bulk_emails_for_compose(emails);
+
+    return has_valid_emails;
+}
+
+export function handle_narrow_activated(filter) {
+    if (should_expand_pm_list(filter)) {
+        expand();
+    } else {
+        close();
+    }
+}
+
+export function handle_narrow_deactivated() {
+    close();
+}