types: Add EditHistoryEvent and APIEditHistoryEvent types.

These types will help make iteration on this code easier.

Note that `user_id` can be null due to the fact that
edit history entries before March 2017 did not log
the user that made the edit, which was years after
supporting topic edits (discovered in test deployment
of migration on chat.zulip.org).

Co-authored-by: Lauryn Menard <lauryn.menard@gmail.com>

zerver/lib/actions.py

@@ -161,7 +161,6 @@ from zerver.lib.string_validation import check_stream_name, check_stream_topic
 from zerver.lib.timestamp import datetime_to_timestamp, timestamp_to_datetime
 from zerver.lib.timezone import canonicalize_timezone
 from zerver.lib.topic import (
-    LEGACY_PREV_TOPIC,
     ORIG_TOPIC,
     RESOLVED_TOPIC_PREFIX,
     TOPIC_LINKS,
@@ -174,7 +173,12 @@ from zerver.lib.topic import (
     update_messages_for_topic_edit,
 )
 from zerver.lib.topic_mutes import add_topic_mute, get_topic_mutes, remove_topic_mute
-from zerver.lib.types import ProfileDataElementValue, ProfileFieldData, UnspecifiedValue
+from zerver.lib.types import (
+    EditHistoryEvent,
+    ProfileDataElementValue,
+    ProfileFieldData,
+    UnspecifiedValue,
+)
 from zerver.lib.upload import (
     claim_attachment,
     delete_avatar_image,
@@ -6695,7 +6699,7 @@ def do_update_message(
         "rendering_only": False,
     }
 
-    edit_history_event: Dict[str, Any] = {
+    edit_history_event: EditHistoryEvent = {
         "user_id": user_profile.id,
         "timestamp": event["edit_timestamp"],
     }
@@ -6869,7 +6873,7 @@ def do_update_message(
         event[ORIG_TOPIC] = orig_topic_name
         event[TOPIC_NAME] = topic_name
         event[TOPIC_LINKS] = topic_links(target_message.sender.realm_id, topic_name)
-        edit_history_event[LEGACY_PREV_TOPIC] = orig_topic_name
+        edit_history_event["prev_subject"] = orig_topic_name
 
     update_edit_history(target_message, timestamp, edit_history_event)
 
@@ -6879,16 +6883,14 @@ def do_update_message(
         assert stream_being_edited is not None
 
         # Other messages should only get topic/stream fields in their edit history.
-        topic_only_edit_history_event = {
-            k: v
-            for (k, v) in edit_history_event.items()
-            if k
-            not in [
-                "prev_content",
-                "prev_rendered_content",
-                "prev_rendered_content_version",
-            ]
+        topic_only_edit_history_event: EditHistoryEvent = {
+            "user_id": edit_history_event["user_id"],
+            "timestamp": edit_history_event["timestamp"],
         }
+        if topic_name is not None:
+            topic_only_edit_history_event["prev_subject"] = edit_history_event["prev_subject"]
+        if new_stream is not None:
+            topic_only_edit_history_event["prev_stream"] = edit_history_event["prev_stream"]
 
         messages_list = update_messages_for_topic_edit(
             acting_user=user_profile,