subs: Use hash_util to get stream id.

Active stream's hash look like this - `#streams/1/announce`
Since the stream_id is present where typically hash section
is for all other hashes, get_current_hash_section should return
the same result.

frontend_tests/node_tests/subs.js

@@ -15,13 +15,13 @@ const ui = mock_esm("../../static/js/ui", {
     get_scroll_element: (element) => element,
 });
 
-set_global("location", {
-    hash: `#streams/${denmark_stream_id}/announce`,
-});
 mock_esm("../../static/js/hash_util", {
     by_stream_uri: () => {},
+    get_current_hash_section: () => denmark_stream_id,
 });
 
+set_global("page_params", {});
+
 const stream_data = zrequire("stream_data");
 const subs = zrequire("subs");