topic -> subject: Extract get_topic_from_message_info().

This changes files where it's safe to just assume caller
may use either "topic" or "subject", and we prefer "topic"
but support "subject".

zerver/tests/fixtures/narrow.json

@@ -36,7 +36,14 @@
             {
                 "message":{
                     "type":"stream",
-                    "subject":"bark"
+                    "subject":"BarK"
+                },
+                "flags":null
+            },
+            {
+                "message":{
+                    "type":"stream",
+                    "topic":"bark"
                 },
                 "flags":null
             }
@@ -54,6 +61,13 @@
                     "subject":"play with tail"
                 },
                 "flags":null
+            },
+            {
+                "message":{
+                    "type":"stream",
+                    "topic":"play with tail"
+                },
+                "flags":null
             }
         ],
         "narrow":[