refactor: Extract message_helper.process_new_message.

We move the message_store.add_message_metadata function
(and all its dependencies) into a new module called
message_helper and rename the function to process_new_message.
(It does a bit more than adding message metadata, such
as updating our message store.)

We also have a "protected" interface now in message_store,
so that message_helper can access the message store:

    update_message_cache
    get_cached_message

Because update_message_cache is identical to
the former create_mock_message, we just renamed it
in the tests.

Most callers should use these functions:

    message_helper.process_new_message (setting)
    message_store.get (getting)

It's slightly annoying that the setter interface
is in a different module than the getter interface,
but that's how you break a bunch of dependencies.

We also extract the tiny message_user_ids class:

    user_ids()
    add_user_ids()

All the code moves here are pretty trivial, and
the code that was moved maintains 100% line
coverage.

The module name `message_helper` is not ideal, but it's a single
function and it'll save time to just do the topology change now and
leave thinking through the right name to later.

static/js/message_store.js

@@ -1,42 +1,23 @@
-import * as alert_words from "./alert_words";
 import * as blueslip from "./blueslip";
 import * as message_list from "./message_list";
 import * as people from "./people";
 import * as pm_conversations from "./pm_conversations";
-import * as recent_senders from "./recent_senders";
-import * as stream_topic_history from "./stream_topic_history";
-import * as util from "./util";
 
 const stored_messages = new Map();
 
-/*
-    We keep a set of user_ids for all people
-    who have sent stream messages or who have
-    been on PMs sent by the user.
+export function update_message_cache(message) {
+    // You should only call this from message_helper (or in tests).
+    stored_messages.set(message.id, message);
+}
 
-    We will use this in search to prevent really
-    large result sets for realms that have lots
-    of users who haven't sent messages recently.
-
-    We'll likely eventually want to replace this with
-    accessing some combination of data from recent_senders
-    and pm_conversations for better accuracy.
-*/
-const message_user_ids = new Set();
+export function get_cached_message(message_id) {
+    // You should only call this from message_helper.
+    // Use the get() wrapper below for most other use cases.
+    return stored_messages.get(message_id);
+}
 
 export function clear_for_testing() {
     stored_messages.clear();
-    message_user_ids.clear();
-}
-
-export function user_ids() {
-    return Array.from(message_user_ids);
-}
-
-export function create_mock_message(message) {
-    // For use in tests only. `id` is a required field,
-    // everything else is optional, as required in the test.
-    stored_messages.set(message.id, message);
 }
 
 export function get(message_id) {
@@ -149,73 +130,6 @@ export function update_booleans(message, flags) {
     message.alerted = convert_flag("has_alert_word");
 }
 
-export function add_message_metadata(message) {
-    const cached_msg = stored_messages.get(message.id);
-    if (cached_msg !== undefined) {
-        // Copy the match topic and content over if they exist on
-        // the new message
-        if (util.get_match_topic(message) !== undefined) {
-            util.set_match_data(cached_msg, message);
-        }
-        return cached_msg;
-    }
-
-    message.sent_by_me = people.is_current_user(message.sender_email);
-
-    people.extract_people_from_message(message);
-    people.maybe_incr_recipient_count(message);
-
-    const sender = people.get_by_user_id(message.sender_id);
-    if (sender) {
-        message.sender_full_name = sender.full_name;
-        message.sender_email = sender.email;
-    }
-
-    // Convert topic even for PMs, as legacy code
-    // wants the empty field.
-    util.convert_message_topic(message);
-
-    switch (message.type) {
-        case "stream":
-            message.is_stream = true;
-            message.stream = message.display_recipient;
-            message.reply_to = message.sender_email;
-
-            stream_topic_history.add_message({
-                stream_id: message.stream_id,
-                topic_name: message.topic,
-                message_id: message.id,
-            });
-
-            recent_senders.process_message_for_senders(message);
-            message_user_ids.add(message.sender_id);
-            break;
-
-        case "private":
-            message.is_private = true;
-            message.reply_to = util.normalize_recipients(get_pm_emails(message));
-            message.display_reply_to = get_pm_full_names(message);
-            message.pm_with_url = people.pm_with_url(message);
-            message.to_user_ids = people.pm_reply_user_string(message);
-
-            process_message_for_recent_private_messages(message);
-
-            if (people.is_my_user_id(message.sender_id)) {
-                for (const recip of message.display_recipient) {
-                    message_user_ids.add(recip.id);
-                }
-            }
-            break;
-    }
-
-    alert_words.process_message(message);
-    if (!message.reactions) {
-        message.reactions = [];
-    }
-    stored_messages.set(message.id, message);
-    return message;
-}
-
 export function update_property(property, value, info) {
     switch (property) {
         case "sender_full_name":