refactor: Extract bulk functions to add/remove peers.

We also streamline some of the error handling code
by doing everything up front.  This will prevent
scenarios where a single bad stream_id/user_id causes a
bunch of the same warnings in an inner loop.

static/js/stream_data.js

@@ -225,6 +225,25 @@ exports.get_sub_by_id = function (stream_id) {
     return subs_by_stream_id.get(stream_id);
 };
 
+exports.validate_stream_ids = function (stream_ids) {
+    const good_ids = [];
+    const bad_ids = [];
+
+    for (const stream_id of stream_ids) {
+        if (subs_by_stream_id.has(stream_id)) {
+            good_ids.push(stream_id);
+        } else {
+            bad_ids.push(stream_id);
+        }
+    }
+
+    if (bad_ids.length > 0) {
+        blueslip.warn(`We have untracked stream_ids: ${bad_ids}`);
+    }
+
+    return good_ids;
+};
+
 exports.get_stream_id = function (name) {
     // Note: Only use this function for situations where
     // you are comfortable with a user dealing with an