frontend: Use topic on message.

This seems like a small change (apart from all the
test changes), but it fundamentally changes how
the app finds "topic" on message objects.  Now
all code that used to set "subject" now sets "topic"
on message-like objects.  We convert incoming messages
to have topic, and we write to "topic" all the way up
to hitting the server (which now accepts "topic" on
incoming endpoints).

We fall back to subject as needed, but the code will
emit a warning that should be heeded--the "subject"
field is prone to becoming stale for things like
topic changes.

frontend_tests/node_tests/topic_data.js

@@ -147,10 +147,10 @@ run_test('test_unread_logic', () => {
     assert.deepEqual(history, ['toPic1', 'topic2']);
 
     const msgs = [
-        { id: 150, subject: 'TOPIC2' }, // will be ignored
-        { id: 61, subject: 'unread1' },
-        { id: 60, subject: 'unread1' },
-        { id: 20, subject: 'UNREAD2' },
+        { id: 150, topic: 'TOPIC2' }, // will be ignored
+        { id: 61, topic: 'unread1' },
+        { id: 60, topic: 'unread1' },
+        { id: 20, topic: 'UNREAD2' },
     ];
 
     _.each(msgs, (msg) => {