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_fetch.js

@@ -3,6 +3,7 @@ import $ from "jquery";
 import * as channel from "./channel";
 import {Filter} from "./filter";
 import * as huddle_data from "./huddle_data";
+import * as message_helper from "./message_helper";
 import * as message_list from "./message_list";
 import * as message_scroll from "./message_scroll";
 import * as message_store from "./message_store";
@@ -49,7 +50,7 @@ function process_result(data, opts) {
 
     messages = messages.map((message) => {
         message_store.set_message_booleans(message);
-        return message_store.add_message_metadata(message);
+        return message_helper.process_new_message(message);
     });
 
     // In case any of the newly fetched messages are new, add them to