user muting: Add helpers to filter out muted users.

This will allow us to avoid duplication of array filtering
logic of the form-
`Array.filter((user_id) => !muting.is_user_muted(user_id))` and
`Array.filter((person) => !muting.is_user_muted(person.user_id))`

frontend_tests/node_tests/muting.js

@@ -92,6 +92,30 @@ test("add_and_remove_mutes", () => {
     assert(!muting.is_user_muted(1));
 });
 
+test("get_unmuted_users", () => {
+    const hamlet = {
+        user_id: 1,
+        full_name: "King Hamlet",
+    };
+    const cordelia = {
+        user_id: 2,
+        full_name: "Cordelia, Lear's Daughter",
+    };
+    const othello = {
+        user_id: 3,
+        full_name: "Othello, Moor of Venice",
+    };
+
+    muting.add_muted_user(hamlet.user_id);
+    muting.add_muted_user(cordelia.user_id);
+
+    assert.deepEqual(
+        muting.filter_muted_user_ids([hamlet.user_id, cordelia.user_id, othello.user_id]),
+        [othello.user_id],
+    );
+    assert.deepEqual(muting.filter_muted_users([hamlet, cordelia, othello]), [othello]);
+});
+
 test("get_mutes", () => {
     assert.deepEqual(muting.get_muted_topics(), []);
     muting.add_muted_topic(office.stream_id, "gossip", 1577836800);

static/js/muting.js

@@ -98,6 +98,18 @@ export function is_user_muted(user_id) {
     return muted_users.has(user_id);
 }
 
+export function filter_muted_user_ids(user_ids) {
+    // Returns a copy of the user ID list, after removing muted user IDs.
+    const base_user_ids = [...user_ids];
+    return base_user_ids.filter((user_id) => !is_user_muted(user_id));
+}
+
+export function filter_muted_users(persons) {
+    // Returns a copy of the people list, after removing muted users.
+    const base_users = [...persons];
+    return base_users.filter((person) => !is_user_muted(person.user_id));
+}
+
 export function get_muted_users() {
     const users = [];
     for (const [id, date_muted] of muted_users) {