markdown: Include text & url in topic_links parameter of our API.

The linkifier code now includes both the shortened text and the expanded
URL, sorted by the order of the occurrence in a topic. This list is passed
back in the `topic_links` parameter of the /messages and the /events APIs.

topic_links earlier vs now:

earlier: ['https://www.google.com', 'https://github.com/zulip/zulip/32']

now: [{'url': 'https://www.google.com', 'text': 'https://www.google/com},
      {'url': 'https://github.com/zulip/zulip/32', 'text': '#32'}]

Similarly, the topic_links local echo logic in the frontend now returns
back an object.

Fixes: #17109.

zerver/lib/event_schema.py

@@ -337,6 +337,13 @@ muted_topics_event = event_dict_type(
 )
 check_muted_topics = make_checker(muted_topics_event)
 
+_check_topic_links = DictType(
+    required_keys=[
+        ("text", str),
+        ("url", str),
+    ]
+)
+
 message_fields = [
     ("avatar_url", OptionalType(str)),
     ("client", str),
@@ -353,7 +360,7 @@ message_fields = [
     ("sender_id", int),
     ("stream_id", int),
     (TOPIC_NAME, str),
-    (TOPIC_LINKS, ListType(str)),
+    (TOPIC_LINKS, ListType(_check_topic_links)),
     ("submessages", ListType(dict)),
     ("timestamp", int),
     ("type", str),
@@ -1380,7 +1387,7 @@ update_message_topic_fields = [
     ),
     ("stream_id", int),
     ("stream_name", str),
-    (TOPIC_LINKS, ListType(str)),
+    (TOPIC_LINKS, ListType(_check_topic_links)),
     (TOPIC_NAME, str),
 ]