subscribers: Extract subscriber_api.

This simplifies some of our dependencies.

As an example, we really don't want compose.js
to depend on stream_subscribers_ui.js, since
the former doesn't use any actual UI code from
the latter.

We also rename the two functions here:

    invite_user_to_stream -> add_user_ids_to_stream
    remove_user_from_stream -> remove_user_id_from_stream

(The notion of "inviting" somebody to a stream is
somewhat misleading, since there is really no invitation
mechanism; you just add them.)

Apart from naming changes this is a verbatim code move.

Finally, we eliminate a little bit of test cruft--the
`override` helper already ensures that a function gets
called at least once during a test.

static/js/subscriber_api.js

@@ -0,0 +1,33 @@
+import * as channel from "./channel";
+
+/*
+    This module simply encapsulates our legacy API for subscribing
+    or unsubscribing users from streams. Callers don't need to
+    know the strange names of "subscriptions" and "principals",
+    nor how to JSON.stringify things, nor the URL scheme.
+*/
+
+export function add_user_ids_to_stream(user_ids, sub, success, failure) {
+    // TODO: use stream_id when backend supports it
+    const stream_name = sub.name;
+    return channel.post({
+        url: "/json/users/me/subscriptions",
+        data: {
+            subscriptions: JSON.stringify([{name: stream_name}]),
+            principals: JSON.stringify(user_ids),
+        },
+        success,
+        error: failure,
+    });
+}
+
+export function remove_user_id_from_stream(user_id, sub, success, failure) {
+    // TODO: use stream_id when backend supports it
+    const stream_name = sub.name;
+    return channel.del({
+        url: "/json/users/me/subscriptions",
+        data: {subscriptions: JSON.stringify([stream_name]), principals: JSON.stringify([user_id])},
+        success,
+        error: failure,
+    });
+}