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>

tools/linter_lib/custom_check.py

@@ -13,6 +13,7 @@ from zulint.custom_rules import Rule, RuleList
 FILES_WITH_LEGACY_SUBJECT = {
     # This basically requires a big DB migration:
     "zerver/lib/topic.py",
+    "zerver/lib/types.py",
     # This is for backward compatibility.
     "zerver/tests/test_legacy_subject.py",
     # Other migration-related changes require extreme care.
@@ -229,7 +230,7 @@ python_rules = RuleList(
     rules=[
         {
             "pattern": "subject|SUBJECT",
-            "exclude_pattern": "subject to the|email|outbox",
+            "exclude_pattern": "subject to the|email|outbox|edit_history_event",
             "description": "avoid subject as a var",
             "good_lines": ["topic_name"],
             "bad_lines": ['subject="foo"', " MAX_SUBJECT_LEN"],