stream edit: Extract stream_subscribers_ui.js.

This is a fairly straightforward extraction.

It's good to test this with Iago, and then go into
Manage Streams and add/remove subscribers for a stream
like devel.

I copy/pasted two small functions that will soon
diverge from stream_edit. The get_stream_id function
will either use a module variable (since we're
generally only editing subscribers for one stream, and
we already have the singleton assumption with
`input_pill`) or a more strict CSS selector.  And then
get_sub_for_target depends on get_stream_id. We may not
always need full subs, anyway, and when we adapt some
of this code for creating streams, things are likely to
change.

I stopped exporting a couple functions that have no
callers outside of this module.

The main entry point for the module is
enable_subscriber_management.

We continue to export invite_user_to_stream and
remove_user_from_stream, which should possibly be just
pulled into their own module to lessen some
dependencies, but they don't have too much baggage,
since they just wrap channel calls.

static/js/ui_init.js

@@ -81,6 +81,7 @@ import * as stream_data from "./stream_data";
 import * as stream_edit from "./stream_edit";
 import * as stream_list from "./stream_list";
 import * as stream_settings_ui from "./stream_settings_ui";
+import * as stream_subscribers_ui from "./stream_subscribers_ui";
 import * as timerender from "./timerender";
 import * as tippyjs from "./tippyjs";
 import * as topic_list from "./topic_list";
@@ -568,6 +569,7 @@ export function initialize_everything() {
     initialize_kitchen_sink_stuff();
     echo.initialize();
     stream_edit.initialize();
+    stream_subscribers_ui.initialize();
     stream_data.initialize(stream_data_params);
     pm_conversations.recent.initialize(pm_conversations_params);
     muted_topics.initialize();