narrow_state: Use message_lists.current to return current filter.

It doesn't make sense for us to track a separate current filter when
it should just the be filter of current message list if there is one.

This will reduce possible confusion in the codebase where filter
returned by narrow_state is different from message_lists.current.

web/src/message_lists.ts

@@ -54,7 +54,7 @@ export type MessageList = {
 export let home: MessageList | undefined;
 export let current: MessageList | undefined;
 
-function set_current(msg_list: MessageList | undefined): void {
+export function set_current(msg_list: MessageList | undefined): void {
     // NOTE: Use update_current_message_list instead of this function.
     current = msg_list;
 }