messages: Use stream_id instead of stream name.

This should cause no functional changes.

This is part of a multi-step effort to move away
from using stream names to reference streams, now
that it's impossible for a user to write a message
with an invalid stream name (since switching to
the dropdown).

web/src/stream_data.ts

@@ -258,6 +258,10 @@ export function get_stream_id(name: string): number | undefined {
     return stream_id;
 }
 
+export function get_stream_name_from_id(stream_id: number): string {
+    return get_sub_by_id(stream_id)?.name ?? "";
+}
+
 export function get_sub_by_name(name: string): StreamSubscription | undefined {
     // Note: Only use this function for situations where
     // you are comfortable with a user dealing with an