compose_state: Use stream_name() and topic() universally.

This change ensures that every call to find the stream name or topic
in the composebox is calling compose_state functions instead of directly
taking a value from an HTML element specified by a classname.

This has better code readability and abstracts away the classname,
and also makes it easier to change which classname we use (which
will be happening in an upcoming change as part of switching the
stream name field to a dropdown).

Note that this change means that the stream name will always be
trimmed, whereas before it was whatever the user had written even
if it had trailing whitespace. This shouldn't be an issue as far
as we (me + Tim) can tell, and also it will become irrelevant as
soon as the dropdown changes land.

static/js/compose_actions.js

@@ -268,8 +268,8 @@ export function update_placeholder_text() {
 
     const opts = {
         message_type: compose_state.get_message_type(),
-        stream: $("#stream_message_recipient_stream").val(),
-        topic: $("#stream_message_recipient_topic").val(),
+        stream: compose_state.stream_name(),
+        topic: compose_state.topic(),
         private_message_recipient: compose_pm_pill.get_emails(),
     };